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Python 1 Instructions
1. Introduction to Python
1.1. The print statement
This first lesson introduces the types of things you'll learn in lessons 2-10
such as printing information to the screen, and getting the user to input
information to the computer. Let's get started!
The first super cool thing we are going to do in Python is print out our name
in the console using the print statement.
A print statement looks like this:
print("Hello,world!")
This code prints the message "Hello, world!" in the console. OK so maybe
this isn't super exciting, but you have to start somewhere!
When typing this code you need to type accurately; your code won't work if
you leave out the () or ""
So be careful!
1. Write a print statement in the code editor that says "Hello, [your
name]!" e.g. "Hello, Monty!"
2. Click to test your code.
3. Click to see if you have passed the task.
1.2. Printing numbers
We can print text to the screen using the print statement. In programming
these chunks of text are called strings, but we'll learn more about that later.
We can also print numbers and math using print statements:
print(5)
print(2+2)
When we print numbers, we still need the brackets () but we don't need
the quotes "" - they are just for strings.
Let's try it!
1. Click to see what the code in the editor prints out.
2. On line 2, change the print statement so that it uses the addition
operator and prints out 8.
3. On line 3, write a print statement that uses the divided by
operator: / and prints out 4.
4. On line 4, add a print statement that uses multiplication: *.
1.3. The input statement
To be honest, only printing stuff on a screen is kind of boring...
We can make our code lots more fun by asking the user questions and
printing their answers. To do this we use an input statement and a thing
called a variable to store their response.
The following code asks the user's age and stores it (using =) in a variable
called age:
age=input("Whatisyourage?")
We can print variables just like strings and numbers:
print(age)
We don't need "" around a variable name either. More on this later!
1. Write a line of code that asks the user's name and stores it in a
variable called name.
2. Display the user's name using the print command.
1.4. Joining data in print statements
We can do more than just printing the variable name. For example, we might
want to ask this: age=input("Howoldareyou?")
But print out:
"You are 16" or "16 is a great age to be!"
There are lots of ways to do this, but let's start easy by 'adding' the parts
together to get the sentences above: print("Youare"+age)
print(age+"isagreatagetobe!")
The + does not add any spaces, so make sure you look closely at your
output to check there are spaces in all the right places!
1. Edit your code from the last task so it prints out "Hello [name]"
2. Add a print statement on line 3 that says "[name] is a nice name!"
Note: [name] means whatever name the user types in, e.g. if the user types
in "Bob" it should print "Hello Bob"; if they type "Gertrude" it should print
"Hello Gertrude".
1.5. Review quiz!
The final task in some lessons is a quick multi-choice review quiz so you can
see how you have understood the content.
Welcome to the first one!
Review Quiz Questions:
1. Which of these is the Python command that lets the user type in
some text?
2. What is the correct symbol for multiplication in Python?
3. Which of these print statements will print out the number 7?
4. The word "Sally" is saved in a variable called name. Which of these
print statements will correctly print out: "That snake is named Sally"
2. Strings and print statements
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2.1. The length of a piece of string
This lesson will give you more practice at using the print statement with
strings.
Lesson 3 will focus more on numbers and calculations
So, what is a string? A string is literally just a bunch of letter, number or
punctuation characters all "strung" together.
In Python, strings are written inside quotation marks: "". You can use
double or single quotes (''), but you should stick to one type throughout
the same program (with a couple of exceptions). Let's use "" for now.
We can add strings to join them: print("Hello"+"Bob")
And we can also multiply them! print("Hello"*5)
1. Click to see what the code does.
2. Delete the existing code and print your name on line 1.
3. On line 2, combine these two strings into one print statement:
"Monty Python ", "and the Holy Grail".
4. On line 3 print the word "Spam!" 8 times.
5. and your code!
Fun fact: The word spam is used to describe annoying emails we don't want
because of this scene from Monty Python!
Monty Python - Spam
2.2. Choose the right quotes!
What happens if we want to print a sentence that has quotes in it? If it's ' it
will be fine:
print("I'mprogramminginPython!")
Strings with a " in them give us an error because Python thinks the second "
is the end of the string:
print("Pythonwasnamedafter"MontyPython"")
We can use '' in this situation to fix the problem:
print('Pythonwasnamedafter"MontyPython"')
What if we have both ' and "? We can put one type of quotes around the
outside of the string, and tell the computer to ignore any more of these by
putting a backslash \ before them. This is called escaping:
print("Andthecomputersaid:\"I'macomputer,silly!\"")
Whenever possible you should use the other quote character around the
outside of a string to avoid the need to \ escape within the string.
Use the guidelines above to complete the following tasks.
1. Choose the right quotes for the quote in the print statement on
line 1 in the code editor.
2. Choose the right quotes and print the sentence: We are no longer
the knights who say "ni", we are now the knights who say
"ekki-ekki-ekki-pitang-zoom-boing!"
3. Write one more print statement that says: There are 10 types of
people in this world, those who know "binary" and those who
don't.
The backslash key,\, which also has the pipe symbol | is above Enter on US
keyboards, and next to Z for UK keyboards. The forward slash / is beneath
the ?.
2.3. Printing on more than one line
Sometimes we want to print text on more than one line. If you try to hit
enter in the middle of a print statement (on line 1) you'll see that it just
breaks your code.
There are 2 ways we can print multi-line statements. The first is by using
triple " quotes: print("""Hittingenterhere
willnowworkproperly""")
The second is to use a special newline character \n. You might recognize the
backslash, which also tells the program to treat the n differently:
print("Hereistext\nontwolines")
The code above works fine, but is often written like this to make it easier to
read: print("Hereistext\n"
"ontwolines")
1. Click to see what happens.
2. Fix the print statement on line 1 using triple quotes.
3. Now that it works, change it to say "does work"!
4. Use \n to write a multi-line print statement on line 3.
5. your code!
2.4. String variables
We can use what we have learned about strings to store data inside
variables too:
name='Fred'
Right now, the value stored inside the variable called name is a string that
says 'Fred'. So we can say that name is a string variable.
Variables are called variables because the values they store can change
during a program.
1. Have a look at the existing code to see how multi-line strings can be
stored inside variables. Print out the poem variable on line 6.
2. On line 10, create a variable called mood and set it equal to "Happy"
3. Print out the mood variable
4. Use the input statement to ask the user how they are feeling today,
store their answer in the mood variable
5. Print mood again
6. (Optional) Use the + operator to print phrase and mood together in
another print statement
Programs often have variables with different datatypes because like we
have already seen, sometimes we work with text, sometimes with numbers
and other types of data.
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2.5. Review quiz!
Let's see what you have learned in lesson 2.
Review Quiz Questions:
1. What characters go around text to make it a string?
2. Which of these is the 'new line' character for putting print
statements on more than one line?
3. What character would be best to use around a string that says:
Don't click the wrong answer!
4. What character would be best to use around a string that says: A
"string" is a sequence of characters
3. Math calculations and operators
3.1. Math operators
Let's take a closer look at math in Python. Here are our main math
operators:
+Addition
‐Subtraction
*Multiplication
/Division
And we also have ** for powers or exponents. For example, 2
3
would be
written as 2**3
1. Write a print statement that calculates: 2 x 5 + 3.
2. Write a print statement that calculates: 5
4
.
3. Write any print statement that has a result of 12.
3.2. BEDMAS - Order of operations
You might know from math that when you have different operations (+,‐,
/,*) in one equation, they have to be done in a certain order. One common
rule is BEDMAS.
This means that anything inside brackets () is calculated first, followed by
exponents (or powers, **), followed by division / and multiplication *, then
addition + and subtraction ‐: print(3+4*2)
In the above print statement the computer multiplies 4*2 first to get 8,
and then adds 3. This gives a result of 11.
If we want to do 3+4 first, we need to use brackets (): print((3+4)*
2)
It's very important to check you have closed all of the brackets you have
opened once you put brackets inside a print statement!
1. Add brackets to the print statement on line 1 so that the output is 5.
2. Edit the print statement on line 2 so that the power (**) is
calculated before the subtraction (the result should be 16).
3. Write a print statement on line 3 that adds 5 and 2 and then
multiplies the result by 10.
Important note: Multiplication and division have equal priority, so if an
equation just has those you can do it left to right as it doesn't matter which
order you do them in e.g. print(3*4/2)#Equals6
print(4/2*3)#Equals6
This is also true for addition and subtraction.
3.3. Identify different types of numbers
If sequences of characters are called strings, then what do we call numbers?
There are two main types of numbers we'll encounter at this level: integers
and floats.
Integers are whole numbers like 1, 56, -32 and so on.
Floating point numbers or floats have a decimal point in them like 3.2 or
0.1.
We identify what the datatype of a variable is by what is stored inside it; if it
stores an integer, it's an integer variable, if it stores a float, it's a float
variable.
One important thing to know is that if you divide in a calculation, you'll
always get a float back, so 8/4 will give you 2.0.
1. In the code editor you will see 4 variables that each store the result
of a calculation. Print out each variable in a separate print
statement. Look closely at the results.
2. Which variable is an integer?
result_3 and result_4 show us something interesting. In
result_3, for example, 0.2+0.1 should be 0.3, but it isn't
exactly. This is because of how computers deal with floats. You
don't need to know why at this level, it's just good to know that this
happens, and it's not a bug in your code!
3. Create a variable called result_5 and store in it a calculation that
uses at least one float.
4. Print result_5 in another print statement.
But why? It's important to know that these are treated as 2 different types
of data because sometimes we need to do different things with them in a
program. You will learn more about this in later lessons.
3.4. Calculations with variables
Let's try using what we've learned about numbers, print statements and
calculations to do something useful!
A common calculation a program might do is calculate someone's pay
(wages) based on their hourly rate and how many hours they have worked
that week.
1. Create a variable under the relevant comment (see note below)
called hourly_rate and store the number 15.50 in it.
2. Create a variable called hours_worked and store the number 40 in
it.
3. Set a variable called wages equal to the hourly rate multiplied by
the number of hours.
4. Print the result.
5. What type of variable is hourly_rate?
6. What type of variable is wages?
7. What type of variable is hours_worked?
#Hashtag: The lines you see in the editor that start with # are called
comments. These are 'notes' that explain what the code does. They are just
for the humans that are reading the code so the computer ignores them.
More on that in lesson5!
3.5. Python math review quiz
Let's review what we've learned about math in Python!
Review Quiz Questions:
1. Which of these is the correct symbol for doing powers such as 2
3
?
2. Which of the following print statements prints 7?
3. Which of these variables is a float?
4. In which of these calculations does the addition (+) happen first?
4. Combining things in print statements
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4.1. Joining things in a print statement
As you have seen we can add two strings together with the + operator:
print("Hello,"+"world")
We can also add variables that have strings stored in them: print("Hello"
+name)
print(greeting+name)
What happens with numbers?
1. Add the wages variable to the print statement on line 6.
2. Click to see what happens. You will see an error–you don't need to
fix this.
We get an error with + because in Python strings and numbers can't
be added together in this way. We'll learn how to deal with this in
the next task.
3. Click to move on to the next task.
4.2. Joining strings with numbers
One simple way that we can combine strings with numbers or calculations is
to use a comma - , like this:
print("Thisweekyouhaveearned:$",wages)
This will print a space between the two parts automatically.
1. Change the + in the print statement to a , (comma).
2. Click to see the output.
4.3. Joining strings with calculations
This method can also be used to join calculations with strings:
print("5+4=",5+4)
Which will print out:
5+4=9
Remember we can use variables that store numbers anywhere that we
would use numbers:
print("Theareaoftherectangleis:",length*width)
1. Add a calculation on to the end of the print statement on line 2.
2. Do the same for line 3.
3. Add a string to the start of the print statement on line 4 so that it
makes more sense.
4. Use the variables from lines 7‑8 to add a calculation into the print
statement on line 11 for the circumference of a circle.
Notes:
The comment starting with # on line 10 has a hint for the circumference
calculation.
PI is a constant value (it doesn't change) so it is written in UPPERCASE.
Make sure you write it the same way in your code. More on this later!
4.4. Format print statements using the built-in .format() function
A comma (,) is not always the best way to combine strings and numbers in a
print statement, because there is always a space between them and
sometimes (like in the wages example) we don't want a space.
Python has a built-in function called .format() which gives us heaps more
control over how our stuff is printed on the screen.
Here is how .format() is used, and you can click here for an explanation:
print("9*3={}".format(9*3))
1. Use {} and .format() to include wages in the print statement on
line 8.
2. .format() the print statement on line 11.
3. .format() the print statement on line 12.
4. .format() the print statement on line 13.
4.5. Print more interesting statements with .format()
The best part about .format() is that it works with anything, and you can
put the missing information anywhere in the string:
print("Hello{},nicetomeetyou!".format("Bob")) print("The
circumferenceis:{}".format(PI*diameter))
We can also put in more than one bit of data by separating each part with a
,:
print("'Hi{},youearned${}thisweek!".format(name,wages))
print("Theareaofa{}by{}rectangleis:{}".format(length,
width,length*width))
1. Use input() to ask the user for their name and store it in a variable
called name.
2. Ask the user for their age and store it in a variable called age.
3. Print out both variables in a string using .format().
4. Click to test your code.
It's really important to make sure you have closed the print brackets AND
the .format() ones!
5. Comments and variable names
5.1. Write comments for the humans that read your code!
Back in lesson3 we took a quick look at comments. In this lesson we are
going to take a closer look at this and some other conventions for writing
Python.
Conventions in coding are basically a set of guidelines that everyone
agrees on to make it easy to understand code, especially if it was written by
somebody else!
Comments are designed to help humans understand what is going on and
why:
#Storetheuser'schosennameforusethroughoutthegame
user_name=input("Andbywhatnameshallthisbrave
adventurerbeknown?")
Usually we try to keep comments short, simple and necessary. Here's an
example of an unnecessary comment: #Sethourly_rateequalto15
hourly_rate=15
1. Take a look at the code in the code editor. Add a # before each line
you think should be a comment then click to check.
2. There is one comment in there that doesn't need to be there, delete
the one that isn't needed.
5.2. Choose good variable names!
A variable can be named almost anything. Python variable names can have
letters, numbers and underscores _ in them.
There are some guidelines to choosing good variable names:
Avoid single letter variable names (most of the time) e.g.: a=15
b=20
c=a*b It is hard to understand what this code does, compared to:
length=15
width=20
area=length*width
Use decent, descriptive names e.g.
hourly_rate=15 can be better than
rate=15 .
Use words from the comments and code in the editor to help you
choose better variable names for these variables:
1. The n variable should be replaced by
2. The s variable should be replaced by
3. The a variable should be replaced by
When you follow these guidelines, your code is easy to understand which
also means you need less comments.
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5.3. Choose even better variable names
Here are some more guidelines for well-named variables:
Variable names can't start with a number.
Variable names should be written as lower_with_under which means
all lower case, with underscores _ separating words. This is compared to
camelCase which is used in other languages:
#Python
first_name="Bob"
#JavaScript
firstName="Bob"
1. Change the variable name on line 2 from camelCase to
lower_with_under.
2. Do the same for the variable on line 3.
3. Do the same for the variable on line 4.
4. Now fix up the variables in the print statement on line 7.
When developers follow the guidelines for a language it makes it much
easier to work in teams where many different people work on the same
code.
5.4. Write all that punctuation so it's easy to read
Python has guidelines for all that punctuation too! Here's a few tips to make
your code more readable:
Don't put spaces before or after brackets: print("Hello,world!"
) is hard to read compared to: print("Hello,world!")
Do put spaces between numbers and operators in calculations, on
either side of equals=, and after commas,: print("2+2‐1=",2+2
‐1) is easier to read than print("2+2‐1=",2+2‐1)
Don't use brackets when you don't need them: print((3*21)+4) -
the * already happens first.
1. Fix up the calculation in the variable on line 1 in the editor.
2. Fix up the print statement on line 2.
My hovercraft is full of eels
5.5. Python conventions quiz!
Let's see how much you have absorbed about how to write good Python
code!
Review Quiz Questions:
1. Which symbol is the symbol for writing comments in Python?
2. If you wanted to store the number of students present in your class
today, which would be the best 'Pythonic' variable name for it?
3. Which of these print statements is written most correctly?
4. Which of these is a valid Python variable name?
6. Turtle drawing basics
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6.1. Meet Tia the Turtle
We're going to take a break from print, input and math for a little while to
play with some Turtles!
Turtle graphics are a bit of a programming tradition and they really help to
understand the idea of sequence in programming.
The example code shows you how to create a turtle called tom by first
importing the turtle module.
1. In the code editor import the turtle module.
2. On line 2 create a turtle called tia.
3. Type tia.forward(50) on line 3.
4. Click to see what happens.
6.2. Move and turn Tia
We can move a turtle forwards or backwards using: tom.forward(50)
tom.backward(100)
The number in the brackets is how many pixels (px), or how far, the turtle
will move.
We can also turn a turtle using: tom.left(90)
tom.right(45)
Here, the number is the angle it will turn, in degrees. So 90 will make it turn
a right angle, 180 will turn it to face the opposite direction and 360 will turn
it all the way around.
1. Click to see what the code in the editor does.
2. Complete the code to make tia draw a square.
3. Modify the code so that tia draws a square that is 200px x 200px.
6.3. Change the color and size
Great! So we're drawing with our turtle. We can change the color and size
of the line that gets drawn using: tom.color("red")
tom.pensize(5)
1. On line 3 set the pensize() to 10.
2. Edit your code for the square so that each side is a different color,
in the order: red, green, yellow, blue.
3. Click to see the image!
These commands happen in sequence or we can say they are sequential.
It's very important in programming to be able to break down a problem into
steps that happen in order. And then we have to make sure that those
things do happen in that exact order!
If you change the color or turn in the wrong place, you will end up with a
different result.
6.4. Draw some different shapes
You can draw all kinds of things with turtles, but you have to think carefully
about how to do it.
In this task there are comments to show you where to write the code for
each shape, and these: .goto(x,y),.penup()and.pendown() have
been used so that each shape can be drawn on a different part of the screen,
without messy extra lines connecting them.
Check the reference section for more detailed explanations of these
functions.
NOTE: Use .left() for all of the shapes in this exercise to keep them on the
canvas!
1. Under the correct comment, from line 11, draw a purple rectangle
that is 120 wide and 50 high.
2. Under the correct comment write the code to draw an orange
triangle with sides that are 60px.
6.5. Filling shapes with colors
We can also fill shapes with a color. There are 3 commands we need for
this: tina.fillcolor() sets the color for filling. tina.begin_fill() goes
just before the line of code that starts drawing your shape.
tina.end_fill() goes right after the last line of code used to draw the
shape.
1. Click to see what the code in the editor does.
2. Set the pen size on line 5 to 8.
3. Set the fillcolor() to yellow on line 8.
4. Add begin_fill() on line 9 - we don't need anything in these
brackets.
5. Add end_fill() on line 18.
6. Click again if you closed your canvas.
7. Introduction to variables
7.1. Calculate donut orders... Mmmm... Donuts....
Variables are a really important part of programming, because they make it
easy to change your code later on.
Imagine if you had a big pay program with the workers' hourly rate of $15/hr
written in there 50 times as a number: worker1_pay=15*40
worker2_pay=15*35
worker3_pay=15*27...
If their pay was increased to $17/hr you would have to change
all 50 of them
individually!
worker1_pay=17*40
worker2_pay=17*35
worker3_pay=17*27...
If we use a variable, we only have to change the number once, then
anywhere its name is written, the new value will be "plugged in":
hourly_rate=17
worker1_pay=hourly_rate*40
worker2_pay=hourly_rate*35
worker3_pay=hourly_rate*27...
1. Read the comments in the code, then fill in each ?? in the
calculation on line 4 with the correct number of each donut to work
out what order #1 costs.
2. Write the correct calculation for order #2 on line 7.
3. Use {} and .format() to output the cost of order 1 on line 10.
4. Add a formatted print statement for order #2 after order #1.
7.2. Use variables to make code more flexible
It works fine if we just write the numbers in there to do the calculations, but
it's not very flexible. Programmers call this hard coding.
If the price of the donuts changes, we have to change lots of numbers for
our code to work.
If we make a mistake while doing this, it's a lot harder to see where the
mistake is...
Do we have the wrong number of glazed or filled donuts?!
Let's refactor this code by creating some variables to store the price of each
type of donut!
1. Which names would be best for these variables:
2. On lines 2‑4, create variables to store the price of each type of
donut using the names you have selected.
3. In each calculation, change the prices to the appropriate variables.
4. (Optional) Try changing the order summary output to a single
multiline print statement with both order costs.
7.3. Write a program for game purchases using variables
Now it's your turn! You've had some good practice at creating variables,
doing calculations with them, and outputting the result to the screen, so
let's try another example.
C.A. Games Sale!
The local gaming store C.A. Games is having a crazy sale on Playstation
games! The cost of games is as follows:
Playstation 3 games: $20
Playstation 4 games: $45
Let's get coding! Use the comments to help you.
1. Create variables for each price you need to store. Use ps3_game
and ps4_game as the names for the variables.
2. Write a calculation for each of orders 1-2. Store the costs in
variables called order1_price and order2_price.
3. Print out each order total along with which order it is e.g. "Order 1
costs: $10". Use a separate print statement for each order.
4. (Optional) As an added bonus, you get an extra $10 off your total
order if you buy 3 or more of the same type of game (e.g. PS3).
This discount can only be applied once to an order. Include the
discount in the calculation for any order that is eligible.
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7.4. Ask for user input to make the program interactive
Now our program is more flexible - we can update the price of any game
type by changing just one number.
It would be nicer if we could type in the number of each game instead of
leaving it hard coded though, e.g.: num_ps2_games=input("Howmany
PS2games?:")
There's one important thing here - if you use input you always get a
string. We're going to need the int() function we saw earlier:
num_ps2_games=int(input("HowmanyPS2games?:"))
And then our calculations can use both variables: ps2_total=
num_ps2_games*ps2_game
1. Use int() and input() to ask the user how many of each type of
game have been purchased, store those values in variables called
num_ps3_games and num_ps4_games.
2. For each type of game, calculate the total cost and store in a
variable. Use ps3_total and ps4_total as the variable names.
3. Calculate the total cost for the order and store in a variable called
total_cost.
4. Print out the total cost for the order in the sentence: "Your order
costs: ${}"
5. (Optional) Try print out a summary of each game type after the
total cost - you can choose which data to include.
Watch your brackets!
7.5.
Review Quiz Questions:
1. Which of the following input statements will correctly take in a
number that can be used in a calculation?
2. Why are well-named variables important in programming?
3. If book_price=5.50 and the deal is "buy 2 get the 3
rd
half price",
which calculation will give the correct total_price?
4. In the following code, what would be the best names for the
variables labelled a,b and the number 15 (in that order)?
8. Numbers and variables
8.1. Increment (increase) an integer variable using +=
We've actually learned a lot about working with string variables, so now let's
take a closer look at working with integer and float variables.
If we have a number stored in a variable there are lots of ways we might
want to change it, but a common way is that we might want to increment,
or add to it: my_var=10
my_var=my_var+10#Nowmy_var=20
So we're adding 10 to the number and storing it back in my_var. In Python
there is a nice shorthand for this: my_var+=10 where the += operator does
exactly the same thing.
1. Create a variable called num_pies and set it equal to 0.
2. Print num_pies in the sentence: "Lisa has {} pies".
3. Use += to give Lisa 5 pies.
4. Print the num_pies sentence again.
Easy as pie right?
8.2. Increment a variable, using another variable
Let's get the user in on the action this time.
Remember we have to use int() if we want to use their input as an integer
rather than a string. There is also a function called float() which you can
use if you think their input might have a decimal point in it (like if it's
money).
start_value=10
amount_to_add=int(input("HowmuchshouldIaddto
thenumber10?"))
start_value+=amount_toadd
1. Ask the user how old they are and store it in an appropriately
named variable.
2. Ask them how many years they would like to add.
3. Increment the age variable using their input.
4. Print out the result in the format "In {} years you will be {}".
5. Add in a few concise comments where you think they are most
needed.
Important note about int() and float().
8.3. Use other operators to change a variable
Awesomely enough, Python doesn't just come with += to easily change a
variable, we also get ‐=, *= and /=:
my_var=15 stores the value 15 in a variable called my_var
my_var‐=5 subtracts 5 and stores the result back in my_var
my_var/=2 divides by 2 and stores it back in my_var
my_var*=4 multiplies by 4 and stores back in my_var
1. What is the value of my_var in the example instruction code above
after all 3 changes?
2. In the code editor, subtract 10 from number1 and print the result.
3. Multiply number2 by 4 and print the result.
4. Divide number3 by 8 and print the result.
8.4. Calculate discounts using shorthand operators
Let's try this with some context by going back to our C.A. Games Sale.
These operators are useful if we want to work out something like a
discount, say $5 off: price=15
price‐=5#Takesoff$5
Or half price: price=15
price/=2#Divideby2andstoreitbackinprice
Or even a 10% discount
(see the note below the tasks for how this is done if
you're unsure about percentages)
: price=15
price*=0.9#10%discount
1. PS3 games have an extra $2 off for one day only, so change the
price using ‐= on line 6.
2. All customers also get 15% off their total order, so change the
total_cost on line 20 before it is printed out.
Calculating a % Discount: One easy way to work out a price after a %
discount is:
1) Write the percentage as a decimal (either in your head or by dividing by
100) e.g. 10%=10/100=0.1
2) Subtract that from 1 e.g. a 10 percent discount is 1‐0.1=0.9
3) Multiply the original price by that number e.g. a 10% discount on $30 is
30*0.9
So in our code we can write price*=0.9 for a 10% discount.
8.5.
Review Quiz Questions:
1. Which operator increments a variable?
2. Which decimal number would calculate the price after a 15%
discount if you multiplied the original price by it?
3. If var_2=4, which of these results in var_2 being 1?
4. What is the value of var_1 at the end of the following code?
9. Debugging print statements
9.1. Hunting for bugs
Another important skill to have in programming is the ability to find
mistakes in your code, or debugging.
You've probably discovered this already if you've had code that has given
you error messages!
So let's go bug hunting...
1. Quotes can be tricky to get right, fix the bug on line 1.
2. See if you can remember how to fix the bug on line 3.
3. There's another error with the quotes on line 5.
4. Good luck finding the problem with line 7!
5. Click to check that all of the code works now.
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9.2. Debug problems with brackets
Brackets were easy when it was just: print("Hello,world!")
But now we have brackets inside brackets inside brackets, so you'll need to
get good at spotting bracket bugs!
1. Find the bug in the print statement on line 1.
2. There's a really common bug on line 3.
3. There are several bugs on line 5 to fix up!
4. Click to check that all of the code works now.
9.3. Debug variables
Variables are case sensitive, which means that my_name, My_Name and
MY_NAME are all different variables.
That's why we try to make them all lower_with_under - it makes the
chances of a mistake lower.
We also have to spell them exactly the same, with underscores in the same
place, any time we use them.
1. Something is wrong with the variable name on line 4, fix it up!
2. Check all the other variable names for bugs and fix them too.
3. Click to check that the code works correctly.
And here's another Monty Python treat - the Knights who say Ni!:
Ekke Ekke Ekke Ekke P'tang Zoo Boing!
9.4. Debug code that uses numbers
Not even computers like it when they can't calculate a math problem!
When writing python code that works with numbers, we have to make sure
we have our order of operations correct.
We also have to make sure that if a number is coming from an input that we
convert it using int() or float()
1. Click run to see what happens
2. Something is going wrong with the calculation. This TypeError
means we're trying to do maths with the wrong kind of data. See if
you can fix this!
3. Click again. Hmm, that price looks a bit strange!
4. The calculation does not calculate what it is supposed to. Fix the
bug in the calculation on line 8.
9.5. Put it all together for some real-world debugging
Now to put your skills to the test. The code in the editor has bugs of all the
types you just learned about, and it is your job to get the code working!
1. Click to see what happens.
2. Find ALL the bugs!
3. Click again to check it's all working.
10. Review lessons 1-9
10.1. Review print and input statements
In Lessons 1-9 we have covered all the basics of sequential programming in
Python. We can now write code that completes a set task step by step and
know the importance of getting these steps accurate and in the right order!
This lesson will review what you've learned one more time before moving on
to more complex stuff.
A print statement outputs text to the screen. It can contain strings,
numbers, calculations or variables.
An input statement lets the user type in text so that we can store and use
it.
1. Ask the user for their name and store it in a variable.
2. Print just their name to the screen.
3. Ask the user what year they were born in and store it in a variable.
4. Subtract this number from 2017 and store it in another variable.
5. Print the result in the sentence "You will be {} years old this year!"
10.2. Review all that maths!
You've also learned a lot about how math is done in Python, including the
order of operations or BEDMAS/PEMDAS.
Calculations can be printed in a print statement:
print(3*4‐7**(4/2))
Or stored in a variable:
result=10‐4*2
And we can use variables that contain numbers to do calculations: num1=
7
num2=5
print(num1*num2)
1. Print the calculation described in the comment on line 1.
2. On line 6, use a shorthand operator to add 10 to the score
variable, then print it on line 7.
3. Use the variables on lines 10‑11 to calculate the area of a
rectangle inside the print statement on line 13.
4. Ask the user for 2 numbers and print out the sum (+) of them. Use
the comments to lay out your code.
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10.3. Review working with strings
We looked at how to work with strings, including joining them together and
getting the speech marks/quotes right.
print("Yournameis"+name) is used for just 2 parts when they are
strings or variables containing strings.
print("3x4=",3*4) works if you need to join a number or
calculation to the end of a string, but it always adds a space.
print("Here'sanumber:{}.Didyouknow3x3={}?And
here'saword:{}".format(7,3*3,"spaghetti"))
We can use .format() for everything, but it's particularly useful when we
need to put something in the middle of a string, or more than one thing in
there.
1. Combine the two string variables in the code in one print
statement on line 5 using +.
2. Print out the string "Coding is fun!" 8 times using 1 print
statement on line 8.
3. Include the calculation in the print statement on line 11 using ,.
4. Put the phone variable into the print statement using .format().
10.4. Write good code that works!
Finally, we also learned about writing good Python code by following the
conventions, and how to look for bugs to make sure code works. This
included:
Good variable names - lower_with_under.
No spaces around brackets e.g. print("hello").
Getting the quotes right!
Checking brackets, quotes, spelling of variable names and that we
have used int() if we need a number from an input.
1. Correctly print out: "Python" is a cool programming language on
line 1.
2. Tidy up the code on line 3.
3. Rename the variables on line 6‑7 with more Pythonic names.
4. Now fix the variable names on line 9.
5. Add the missing variables into the .format() on line 11.
6. Fix the bugs on line 11.
7. Click to check it all works!
10.5. Review quiz time!
Time for the last review quiz in this section.
Review Quiz Questions:
1. Which is the better print statement?
2. Which calculation prints 12?
3. What is the value of x at the end of this code?
4. Which input correctly gets a user's favorite number for a program
that will add 10 to it?
11. Introduction to selection
11.1. Introduction to selection
So far we have seen code that runs in sequence. However, often in a
computer program decisions need to be made.
This is called selection in programming - making decisions about what
should happen based on conditions.
In this course, the keywords if and else form our key
selection/conditional structures.
This lesson will introduce you to what will be covered in lessons11-20. Don't
worry if anything seems confusing at this stage, as you'll learn about it all
more in the next lessons. So, let's take a look at some code!
1. Click and type in your name.
2. Click and type in "Monty" with a capital "M".
3. Change line 3 to have your name after the == instead of "Monty".
4. Click again and type in your name.
Important Note: if statements are case sensitive, so that means typing
"bob" is different from typing "Bob" - you'll learn how to deal with this in
lesson12.
11.2. Understand how a condition works
When an if/else statement is written, it always has a condition. That's the
part that gets checked, and then a decision on what to do is made based on
whether the condition evaluates to True or False.
If the condition is True the if branch gets run. The else branch gives a
default case that gets run if the condition is False.
The simplest check is checking whether or not two things are the same with
the is equal to == operator.
If we print out a comparison of some kind, Python will tell us if it's True or
False:
print(4==9) prints out False, but:
print("Bob"=="Bob") prints out True.
1. Click to see these in action.
2. Write a print statement on line 5 that will print True.
3. Write a print statement on line 8 that will print False.
4. Click to check you're correct.
11.3. Use different comparative operators
There are a few different types of comparisons we can make inside
conditions. The symbols we use, like ==, are called comparative operators.
You'll learn more about these in lesson13, but here are a couple more: > is
greater than < is less than
So a condition like if4<9: would evaluate to True because 4 is less
than 9.
Let's try checking some conditions!
1. Change the operator in the print statement on line 2 so that it
prints True.
2. Change the print statement on line 4 so that it prints False.
3. Change the if statement so that the messages work properly.
4. Make sure you your code a couple of times, entering numbers
above and below 10 to check it works!
11.4. Use elif to make more options
So using conditional if/else structures is pretty much like putting a fork
in the road in your program. There are 2 branches and it could go either
way depending on whether the condition is True or False.
Sometimes we might want more than two branches on our path. We can do
this with elif (short for else if), which gives us a chance to add another
condition. Run the example code to see this in action.
In the code editor you will see another comparative operator: <= less than
or equal to. There is also a >= greater than or equal to.
1. Click to see what the code does. Do this 3 times with a number
from 1-10, one from 11-80, and one above 80.
2. Change the <= on line 3 to <, then see what happens if you run it
and type in 10.
3. Put the <= back, then change the condition on line 5 so that
numbers up to 100 are medium numbers
(instead of 80)
.
4. Test the program again with the 3 types of numbers to make sure
it works.
11.5. Review quiz on selection
Let's quickly review the basics so we can get into some more complex stuff!
Review Quiz Questions:
1. Which of these print statements prints True?
2. Which operator means "less than"?
3. Which operator means "greater than or equal to"?
4. Which if condition evaluates to False if the number 16 is stored
inside the variable age?
12. More selection options
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12.1. Get to know the if statement a bit better
The if keyword will make a piece of code run if the condition in it is True.
It's a bit like if someone's parent says: "I'll buy you a new phone if you keep
your room tidy for a month". As long as they meet that condition (tidy your
room), they'll get the new phone.
Look at the example code. An if statement must look just like this with the
word if, a condition and the colon :.
The code that we want to run is indented by pressing tab to show that it is
inside the if. This is called a block.
1. In the code editor there is a : missing, put it where it should go.
2. Indent the block that should be inside the if.
3. Click and type in "happy" to see if it works.
Remember it's case sensitive so "Happy" is different from "happy".
12.2. Add a second if statement for more options
Maybe we want to have 2 different responses. This time you will have a go
at writing a second if statement.
1. On line 5 add another if statement that checks if the user types in
"sad".
2. On line 6 add a block that prints "Sorry to hear that" if they do type
in "sad".
3. Click and test your code by typing in "sad".
4. Click again and test it with "happy" to make sure you haven't
broken the first if statement.
Handy Hint: Whenever you have if statements in your code, it is always
important to test all the different branches or outcomes you are expecting,
just like you did in this task.
12.3. Add a default response using else
If we wanted to check for every possible mood we could think of, we would
need a lot of if statements! Luckily, Python has another handy keyword
called else.
This basically says if it's anything other than the thing we checked for in the
if, then do this e.g. iffave_food=="pizza":
print("Yum!")
else:
print("Yuck")
In this code, if you type "pizza" it will say "Yum!" and if you type anything
else at all it will say "Yuck". This is why else doesn't need a condition.
You can test this out in the example code.
1. Change the second if (on line 7) to say else and delete the
condition.
2. Click and try typing in anything other than "happy".
3. Click and check that "happy" still works as expected.
12.4. Adding more than one condition using elif
Hmm, our code works but it might be useful if we could have a couple more
responses to specific moods, and then let our else run for anything else.
When we want more than one branch with a condition in our code, we use
elif which stands for else if. The elif connects to the if before and the
else after it, making sure that only one of the branches can run.
Click here for an explanation of this code: iffave_food=="pizza":
print("Metoo!")
eliffave_food=="nachos":
print("Thoseareprettygood")
else:
print("Hmm,notoneofmyfavorites")
1. Change the 2
nd
if to an elif to join the statements together.
2. On lines 9‑10 add another elif that prints "You should get an early
night" if the user enters "tired".
3. Below this, add an else that will print "Oh, really?" for all other
cases.
4. Click 4 times and test each branch of your if statement to make
sure they all print the correct message.
12.5. Review quiz
What have you learned about if statements?
Review Quiz Questions:
1. Which character goes at the end of a line of code that starts with
if?
2. What is the keyword used for making a second branch in an if
statement when both branches have a condition?
3. Which operator means is equal to in Python?
4. How do you know which bit of code is inside an if statement?
13. Boolean values and operators
13.1. Recognise and use > and < (greater than and less than)
In this lesson we will look more closely at conditions. Boolean (pronounced
BOO-lee-in) is the name given to these special types of values that are either
True or False.
The code x=False creates a variable called x and stores the value False in
it. We then say that the datatype of x is Boolean, or x is a Boolean variable.
In Python: < means less than; > means greater than. <= means less than
or equal to; >= means greater than or equal to. == means equal to; !=
means not equal to.
The code a=25>12 sets a to True since 25>12 is True (25 IS greater
than 12).
Any calculations are done before things are compared.
13.2. Using and and or keywords
In Python there are some other useful keywords for working with Boolean
values, and and or:
aandb is True if both a and b are True.
aorb is True if either a or b or both are True.
Some examples:
a=8<9and2>=2
This sets a to True since 8<9 is true AND 2>=2 is True.
b=5>2or3==2
This sets b to true since 5>2 is True, even though 3==2 is not. If they
were both False, b would be False.
Note that > and < are evaluated before and and or.
13.3. Use not - not ! but the other NOT...
We also have the word not. This sort of "reverses" whether something is
True or False. So notTrue is False, and notFalse is True.
Here's another example:
not3>4 is True because 3>4 is False and the not reverses it.
We can store Boolean values inside variables too, just using the words True
and False (which must start with a capital in Python): x=True
y=False
Now notx is False, and noty is True.
Note that not is evaluated before and and or.
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13.4. Using operators with strings
You might be wondering what happens if you put text rather than numbers
into these comparisons.
If we use our <,<=,>=,> operators, then Python works out which string comes
first alphabetically based on the first letter, like the letters are numbered 1-
26:
"a"<"b" is True because a is before b in the alphabet
(1 is less than 2).
"Turtle"<"Aardvark" is False because T comes after A in the alphabet
(20 is greater than 1).
We also have a fun operator called in. When used with strings, this will tell
you if a string is in another string:
"a"in"apple" is True, but "B"in"banana" is False - B is not the same
as b.
13.5. Putting it all together
Let's review Boolean values by trying some examples with all of the
operators.
Boolean is the word used to describe values that are either True or False.
Just like strings, integers and floats, if a variable contains a Boolean value,
we call it a Boolean variable.
Remember: <lessthan
>greaterthan
<=lessthanorequalto
>=greaterthanorequalto
==equalto
!=notequalto
and‐bothmustbeTrue
or‐onemustbeTrue
not‐makesittheopposite
We have looked at how these operators store Boolean values in variables,
but they work the same way in if statements as you'll see in upcoming
lessons.
14. Matching strings
14.1. Take a closer look at the == operator
Let's take a closer look at == and how it works with strings.
With numbers, it's pretty easy:
4==4 is clearly True, while 3==27 is clearly False.
With strings it's a bit more complicated. As a human, you might look at the
comparisons below and think they would be True: "Hello"=="Hello"
"Hello"=="hello"
Computers, however, think of capital letters and lower case as different
(case sensitive), and they don't ignore extra spaces like we might.
1. Click and type in "bob" with no capitals to see what happens.
2. Which message is displayed?
3. Click and type in "Bob" to see what happens.
4. Which message is displayed?
5. Click to carry on.
In this lesson we will look at how to work with strings and == so that our if
statements work properly.
14.2. Match strings using the == operator
What does case sensitive mean? It's important to understand what that
means for comparing two strings because it can cause unexpected results if
you get it wrong.
The bottom line is this:
print("Hello"=="Hello") will print True, but:
print("Hello"=="hello") will print False.
Even if all of the letters are the same, if one is a different case the strings
are not the same.
1. Make the print statement on line 2 print True.
2. Replace the ?? to make the print statement on line 3 print True
(Use the same sentence)
.
3. Make the print statement on line 6 print False
4. Replace the ?? to make the print statement on line 7 print False
(Use the same sentence)
.
14.3. Match strings with spaces
Believe it or not, spaces also make a difference when matching strings! So
these 4 strings are all different: "Hello,howareyou?"
"Hello,howareyou?"
"Hello,howareyou?"
"Hello,howareyou?"
The next task will show you how to deal with extra spaces at the beginning
and end, but for now let's check that you've got the idea.
1. Add some spaces to the start or end of the strings on line 2 so that
the print statement prints False.
2. There's a string stored inside the phrase variable but it doesn't
quite match the condition on line 7. Fix up the if statement so it
will print!
3. Make the print statement on line 11 print True.
As you can see on line 7, comparing strings, and variables that contain
strings works exactly the same way:
if"Hello"=="Hello" and ifgreeting=="Hello" and ifgreeting
==other_greeting are all valid conditions.
14.4. Deal with capitals and spaces
Luckily, Python has some built-in functions that help us to deal with capital
letters and spaces so that our code is more robust (doesn't break).
Here are a few, although there are lots more: .strip()#removesall
spacesfromthestartandendofthestring.
.lower()#convertsawholestringtolowercase
.upper()#CONVERTSAWHOLESTRINGTOUPPERCASE
.title()#GivesEachWordACapital
.capitalize()#Makesthefirstletterofthestringacapital
1. Click and type in a word with some spaces at the start and some
capital letters in it to see the result of .lower().
2. Under the relevant comment, use .strip() to remove any spaces.
3. Do the same for .title().
4. Finally, use .upper() to convert the word to upper case.
5. Test your code a few times by clicking and entering a range of
words with and without capitals and spaces.
These functions are important when users are typing text into an input and
you can't control whether they type capitals or spaces.
14.5. Review quiz on comparing strings
Let's review what we've learned about comparing strings!
Review Quiz Questions:
1. Which string is identical to the string in the print statement inside
the if branch?
2. If the user types in "SHRUBBERY", what will be the value stored in
password after line 3?
3. Which of these would not result in "Traitor" being printed?
4. What is printed if the user types "ShRuBbErY"?
15. Numeric input in if statements
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15.1. Use numeric input in if statements
We learned in lessons1-10 that if we want to USE a number that the user
has typed in AS a number, we have to use int() to convert it.
This is also true if we want to use numbers entered by a user in a
comparison.
1. Click and enter your height to see what happens. Type only the
number e.g. 156 not 156cm.
2. Fix up the input statement on line 2 so that the code works.
3. Click again and enter your height to see if you are tall or short.
TypeError:unorderabletypes:'str'>int() This error means that
you can't compare 2 different types of data, in this case the string you
typed in (str) and the integer (int) 165in the condition.
15.2. Write an if/else statement using numbers
Let's try another example so we can practice asking for numeric input and
comparing it.
1. Ask the user how much time they spend on the internet each day
and store it in a well-named variable.
2. Write an if statement that checks if the user spends less than 3
hours online.
3. If the user spends less than 3 hours, print "That's a healthy
amount of time".
4. Otherwise, print "You need to get more fresh air!"
5. Click and test both branches of your if statement.
15.3. Adding more branches with elif
Let's use elif to add some more options based on the user's input. The
code below shows how you could check if a person's age is between 15 and
30: ifage>15andage<30:
print("Youareolderthan15butyoungerthan30")
Remember an and condition is True if both parts are True.
What happens if you type 15 or 30? Because we have used < and >, 15 and
30 (our boundary values) are not included. If we wanted to include them
we would have to write: ifage>=15andage<=30:
1. Add an elif on line 7 that checks if the user spends between 3 and
5 hours online, including 3 and 5.
2. On line 8 make it print out "Make sure you are also being active." if
the elif is run.
3. Read the elif and else statements on lines 9‑12 and replace the
?? with a boundary value that makes sense.
4. Click and test all branches.
5. (Optional) Edit the first if so that when the user enters less than 0
hours, it will run the else branch.
15.4. Make comparisons between 2 stored values
So far we have made comparisons using strings, numbers, and variables
containing a string or number. We can also compare 2 variables:
retirement_age=65
ifage==retirement_age:
print("Youcanretirenow!")
And this will work whether the variables are strings, integers or Boolean,
although they must be the same type if you're using the < or > operators.
1. Store the number 9in the constant on line 2
2. Complete the condition in the if statement on line 8 using the
variable and the constant.
Click here for an explanation of what is meant by a "constant".
15.5. Review Quiz
Time to review if statements with numbers!
Review Quiz Questions:
1. What needs to be added to the input statement for this code to
work?
2. Assuming that bug is fixed, what will be printed if 156 is entered?
3. What will be printed if 175 is entered?
4. What will be printed if -4 is entered?
5. Which of these is a constant?
16. Formatting your code
16.1. Write more good Python code!
There are conventions, or guidelines, for writing conditional code as well in
Python, and this lesson will go over the key points.
In if/elif/else statements, blocks must be indented for code to run
correctly.
You can have as many lines of code as you want inside each branch of your
if statement, as long as they are all indented.
1. In the code editor, lines 8‑10 are indented, but they don't line up.
Indent them all the same distance so the code will work.
2. Lines 13‑14 should also be a part of the else branch. Indent them
correctly so that all 3 print statements are printed when the user
guesses wrong.
3. Click to check it's all working.
16.2. Use correct spacing and punctuation around conditions
Just like when we do print statements, if statements have guidelines about
where to put spaces.
DO put spaces between each part of your condition:
Use ifage>16: rather than ifage>16
Even if it is long do this:
ifage*3>21and"b"in"birthday": not: ifage*3>21and
"b"in"birthday":
Don't put spaces around the colon :
Use ifage>16: rather than ifage>16:
Don't use unnecessary brackets in if statements (commonly done in
other programming languages:
Use: if"a"in"age"and"b"in"birthday": Rather than these: if
("a"in"age")and("b"in"birthday"):
if("a"in"age"and"b"in"birthday"):
1. Fix up the condition in the if statement.
2. Fix up the elif statement.
3. Fix up the else statement.
4. Test all 3 branches of the statement.
16.3. Use constants for values that are set or don't change
We have touched briefly on constants. These are variables that don't
change, and they are an important part of writing good code.
Constants are written in UPPER_WITH_UNDER or caps, with underscores
separating words. They should be used when a value is set up and doesn't
or shouldn't change.
1. Read through the code in the editor carefully. Click to see what it
does.
2. Decide which variables are numbers that are set and don't change
in the program, and rewrite them as constants.
3. Click and check it still works.
NOTE: The comments in this block of code are examples of bad comments -
you will fix these in the next task!
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16.4. Write useful comments
Remember how we talked about writing comments that are short, simple
and necessary? if statements are a common place where people write
unnecessary comments:
#Ifguessequalsanswer
ifguess==ANSWER:
score+=10
This is pointless because if you read the actual line of code below it, you
would say exactly the same thing.
A better approach is to write a comment that explains what the purpose of
that if block is, such as:
#Checkwhethertoaddpointsforacorrectanswer
1. Read the code in the editor and work out what each part does.
2. Select a better comment for line 4:
3. Select a better comment for line 7:
4. Select a better comment for line 10:
5. Select a better comment for line 17:
16.5. Review quiz
What else have you learned about conventions?
Review Quiz Questions:
1. Which if statement is written most correctly?
2. Which is the best comment for these lines of code:
3. How far should code be indented if it's inside a block (e.g. if) in
Python?
4. We've got another program that calculates pay for workers:
everyone works 40 hours/week, some people get paid different
hourly rates, some work overtime (extra) hours. Which value is
most likely to be a constant?
17. Formatting using an index
17.1. Use indices to format strings
We're going to take a short break from if/else structures in order to look at
some more things we can do with .format():
print("Hello,{}!Youmustbe{}!".format(name,mood))
When we use .format() like this, the "missing values" in the brackets (called
arguments or parameters) are inserted in order. But each argument is
numbered from 0, and we can write that number, or index, inside a
placeholder to say where it should go:
print("Hello{1},it's{0}today".format("sunny","Teresa"))
Here the arguments are in the wrong order inside the .format() brackets,
but we can insert them into the correct placeholders by using the index:
"sunny" is 0, and "Teresa" is 1.
1. Fill each {} placeholder in the print statement on line 1 with the
index for the correct word so that the quote makes sense.
2. Do the same for the print statement on line 6.
3. Click to check it works OK!
If you're a bit rusty on .format() you might like to revisit lesson4.
17.2. Using an index more than once
You might be wondering why we wouldn't just write the arguments in order.
A better use for this is when you need to insert the same argument more
than once in a string: print("{0},{0},whereforeartthou
{0}?".format("Romeo","Bob"))
We can use each argument as many times and anywhere we want, as long
as we put numbers in all of the placeholders.
1. Fill in the {} placeholders with the correct indices on line 1.
2. See if you can reconstruct the famous Monty Python quote on line 3
using the index for each word (Google might help if you get stuck).
3. Click to check it's working.
Remember to count from 0!
17.3. Store a string to be formatted in a variable
If you store a string containing the {} placeholder inside a variable, you can
then use .format() straight on the variable:
greeting="Hello{},welcometoTheQuizGame!"
print(greeting.format(name))
This could be useful if you need to use a sentence more than once in a
program, but with different values. It can also be used for constants -
remember they function exactly the same as variables.
1. A template sentence has been stored in the constant on line 2.
Write a print statement on line 10 using .format() that puts the
variables in the right part of the sentence.
2. Click and enter some random words to see if your sentence makes
(grammatical) sense!
17.4. Format floats so they have the right number of decimal places
The last handy tip we're going to learn about .format() for now is how to
make floats look better when we plug them into a string. Run the example
code to remind yourself of what it usually looks like.
In many cases, like money, we might only want 1 or 2 decimal places (dp)
like: $36.75. This is how we control the format:
print("${:.2f}".format(12/7))
This prints out $1.71. The .2 tells us how many decimal places. Click here
for a little more detail.
1. Look at the code in the editor and click to see what it does.
2. Hmm that looks a bit weird. Change the print statement on line 6 so
that it prints out the hours to 1dp.
3. Fill in the {} on line 8 so that the wages are printed to 2dp.
4. Write a print statement on line 14 that prints the wages after tax
with 2dp, in the sentence "After tax: ${}".
17.5. Review quiz!
Hopefully you've got the hang of working with .format() now!
Review Quiz Questions:
1. What is the index of the variable age in this list of arguments (the
bits inside the brackets) below?
2. What does the following statement print?
3. Which statement correctly prints "I like to eat, eat, eat, apples and
bananas" after this line of code?
4. Which of these is the correct placeholder to use if we are formatting
money to 2dp?
18. Debugging if statements
18.1. Bug hunt #2
There can be a lot of pesky little bugs in if/elif/else structures, so you'd
better get some practice at finding them!
1. Find the 1
st
bug in the if/else statement!
2. Find the 2
nd
bug in the if/else statement!
3. You could the code to see what the error message says if you get
stuck. And run it again at the end and test it with the right and
wrong answer.
18.2. Indenting bugs
Python code must be indented correctly in order to run, otherwise you'll
get the dreaded: SyntaxError:unindentdoesnotmatchanyouter
indentationlevel
1. Fix up the indents in the if/else statement!
2. The final print statement printing the score should happen after
the if/else, for any answer the user types in. Fix this bug.
3. Click and test all the branches of the if statement.
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18.3. Greater... or less than? No... definitely greater. I think...
Another common bug is getting < and > mixed up! If we put a greater than
instead of a less than, our code will do exactly the opposite of what we
want it to do!
Did you know that 10 bees weigh approximately the same as 1 M&M candy?
1. Click to run the quiz question code in the editor and test it with a
number higher than 10.
2. Test it again with a number lower than 10. Are these the expected
responses?
3. Fix the 2 bugs in the if/elif/else statement so that the code
works properly.
4. Click again and make sure it works!
18.4. Spell those keywords right
Sometimes it's hard to remember exactly what all the keywords are in a
programming language. Especially if you haven't used the language for a
while, or you learn more than one language!
Whoever wrote this code must have gotten mixed up between Python and
some other languages!
1. Find and fix the incorrect keyword in the conditional part of the
code.
2. There are 2 more lines of code that have not been written in a very
Pythonic way. Find them and fix them up!
3. Click and test the code works
Important note: If you use .strip() and/or .lower() on a variable before
using it in a comparison, make sure you put the lower case form of the
word in the condition! E.g.: ifvariable=="rightanswer": rather than
ifvariable=="RightAnswer")
Same idea if you use: .upper(),.title(),.capitalize() Make sure
you're checking for the right thing!
18.5. Linking ifs with elses
Any time you put in an if keyword it starts a new selection structure. In
other words, you can't have more than 1 if in the same selection code. If
you add an else, it will only be linked only with the if directly above it.
If you need more than 2 options it is really important to use the elif
keyword so that all of the branches are linked together.
1. Click and type in "titan" to see what happens.
2. Fix the keywords so that it will only give us the "correct" message
when "titan" is typed in.
3. Click and check it works!
Click here for an explanation of why this happens.
19. Testing if statement conditions
19.1. Test if statements using ==
In this lesson, you'll test if statement code from the Knights who say "Ni!"
online training program. Anyone who wants to join the order must complete
this induction before actual knighthood training begins.
The application for a knighthood begins by asking the applicant's location.
Training courses start on different dates depending on the location.
1. Click and enter input to run the 1
st
branch of the if statement.
2. Click and run the 2
nd
branch (elif).
3. Click and run the 3
rd
branch.
4. Click and run the else branch.
19.2. Testing if conditions with numbers
The Knights of Ni training program has a weight restriction, so the next
question asks the user's weight.
Note that if you type 75kg you will get an error because 75kg is not a valid
number. Type numbers, e.g. 75 (without kg) to test this code.
1. Click and run the 1
st
branch of the if statement.
2. Click and run the 2
nd
branch.
3. Click and run the 3
rd
branch.
4. Click and run the else branch.
Important note: If you just click OK with this code, your will get an error
because we have not yet learned how to deal with a user typing non-
numerical data into an input with int() around it. Don't panic! We'll get
there.
19.3. Test if statement conditions
The Knights of Ni training course has a height restriction too. They are a
fussy bunch.
1. Click and test the 1
st
branch of the if statement.
2. Click and test the 2
nd
branch.
3. Click and test the 3
rd
branch.
4. Click and test the 4
th
branch.
5. Click and test the else branch.
As before, if you type 1.5m you will get an error because 1.5m is not a valid
number. You must type a 1.5 without the m on the end.
19.4. Test more complex if statement conditions
A potential knight's endurance is tested by their ability to completely weed
and prune a shrubbery.
Shrubs in the southern hemisphere are much tougher and the weeds
grow twice as big, so the time limit is different depending on the applicant's
location.
1. Click and test the 1
st
branch of the if statement.
2. Click and test the 2
nd
branch.
3. Click and test the 3
rd
branch.
4. Click and test the 4
th
branch.
5. Click and test the 5
th
branch.
6. Click and test the else branch.
If you type 10m45s or 10:45 you will get an error because they are not valid
numbers. Type 10.75 instead.
19.5. Test boundary values in if statements
It is important to test boundary values to make sure you have got your
conditions correct.
The easy way to figure out what the boundary values are, is to look at any
ranges or limits you have in your conditions.
In the code in the editor, we have 3 ranges to test: 1-30, 31-129 and 130+.
The boundary values are the numbers on either side of each end of the
ranges - the numbers on the boundaries.
This gives us: 0 and 1, 30 and 31, 129 and 130 as boundary values.
1. Click and test the boundary values for the 1
st
branch.
2. Click and test the boundary values for the 2
nd
branch.
3. Click and test the boundary value for the 3
rd
branch.
Notes: The numbers in your conditions are easy to identify as boundary
values, but the other boundary values will depend on whether you use <,
>,<=,>=,==or!= In an if/elif structure, some of the boundary
values will double up. For example, in this code 31 was a boundary value for
the 1-30 range as well as the 31-129 range, but you only need to test it once.
20. Review lessons 11-19
20.1. Review the if statement
This lesson will review the content you've learned in lessons11-19. For the
first task, let's get you to write some code!
These lessons taught you about selection or conditional structures. They
are called this because they are the decision-makers in a program. They
test out a condition and then run one of several possible branches
depending on whether the condition is met or not.
First let's practice a simple if statement.
1. Ask the user for their favorite ice cream flavor and store it in a well-
named variable.
2. Convert their answer to lower case and strip any spaces.
3. Write an if statement that checks if their favorite flavor is "cookies
and cream" and print "Mine too!" if it is.
4. Click and test that the message prints.
5. (Optional) Add concise comments to your code.
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20.2. Review quiz!
To mix it up a little bit, let's do a quiz to review the basics.
Review Quiz Questions:
1. Which operator means greater than or equal to?
2. What does the or operator mean? For example: x=3>2or9<
6
3. What symbol goes at the end of an if statement to start a block?
4. Which of these if statements is written most correctly?
20.3. Review if/else statements
The else branch, when added to an if statement, gives us a default case. It
gets run if none of the conditions in the if or elif branches are met.
An if statement works fine without an else, but an else must be attached
to an if or if/elif.
Let's write one.
1. Ask the user how many servings of fruit and vegetables they have
had today and store it in a well-named variable.
2. Write an if statement that checks if they have had 5 or more.
3. If they have, print "Well done, you've had your 5+ today!"
4. Write an else branch that says "You should eat 5+ a day, every
day."
5. Test both branches using .
6. (Optional) Test the boundary values, and add concise comments to
your code.
20.4. Conditions review quiz
Let's have another go at working out whether conditions are True or False.
Remember: <Lessthan
>Greaterthan
<=Lessthanorequalto
>=Greaterthanorequalto
==Equalto
!=Notequalto
or‐isTrueifeitherorbothsidesareTrue
and‐isTrueonlyifbothsidesareTrue
not‐reversesite.g.TruebecomesFalse
Review Quiz Questions:
1. Is this condition True or False?
345<274
2. Is this condition True or False if x = True?
3*3>=4+5andx
3. Is this condition True or False if x = False?
9‐7<1ornotx
4. Is this condition True or False if phrase = The noblest of all dogs
is the hotdog; it feeds the hand that bites it?
"c"inphrase
20.5. Review if/elif/else statements
Lastly, let's practice writing an if/elif/else statement one more time.
Remember that if you want all of your branches to be linked together so
that only one is run, you need to use if/elif/else not multiple if
statements. Otherwise the else will only be attached to the if right before
it, and more than one branch will run.
1. Ask the user to rate the last movie they watched out of 5 and store
it in a well-named variable.
2. Write an if statement that prints "Pretty average huh?" if the rating
is 3.
3. Write an elif branch that prints "Wow, that must have been bad!"
if the rating is less than 3.
4. Write another elif that prints "Sounds like a great movie!" if the
rating is more than 3 and less than or equal to 5.
5. Write an else branch that prints "I said out of 5!" for anything else.
Don't forget: It's important to test the boundary values once you start
writing conditional structures, especially ones with multiple branches.
21. Introduction to for loops
21.1. Introduction to loops
In lessons11-20 you learned about selection (also known as conditional)
structures that let your code make decisions.
In lessons21-30 you will learn about iteration or code that repeats. These
structures are also known as loops.
We will look at 2 types of loops: for loops and while loops.
Loops are useful if we want to do something multiple times in a program.
1. Look at the code in the editor and try to guess what it might do.
2. Click and see what it does.
We had to write out that part that asks for the number of hours worked 5
times! If only there was a way to make that easier...
21.2. Meet the for loop
Let's take a look at our first loop... the for loop:
foriinrange(5):
print(i)
A for loop repeats the block of code inside it a set number of times.
i is the built-in counter that keeps track of how many times the loop has
repeated.
1. Run the code in the editor to see how i changes as the loop is run.
2. Change the loop statement so that the numbers 0-9 are printed.
Note that because it starts from 0, the last number printed is always one
less than the number we put in range().
21.3. Customise a for loop
We can customise a for loop in several ways including making it start and
stop at different numbers. You will learn more about this in the next
lesson.
1. Click to see what the code in the editor does.
2. Compare the output to the code and see if you can figure out how it
works. Make sure you don't change these 3 loops.
3. Add a for loop to line 18 where the first number printed is 5 and
the last number printed is 17.
4. Click to see if you got it right!
21.4. Meet the while loop
The while loop can be used in a similar way to a for loop for printing out
numbers, but there are some differences.
In a while loop, instead of saying how many times we want the loop to
repeat, we use a condition -- just like in an if statement. The loop will run
until the condition is met: whilei<=5:
print(i)
This is the basic structure of a while loop, but we have to write a bit more
code for it to work because it doesn't have a built-in counter like the for
loop.
1. Look at the code in the editor and see what we have to do with i to
make it work.
2. Change the code so that the numbers 0-10 are printed.
3. See if you can change the code so that it starts from 1.
4. (Optional) Change the conditional operator so that 10 is NOT
printed.
So we have to set up the counter first, with the start point, and then
increment it inside the loop.
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21.5. Use other conditions in a while loop
Because a while loop runs on a condition, we don't have to use just
numbers. We can write any condition we want in there, but be careful
because something like: while3<4 will make an infinite loop, because it
will never be False, and so won't get switched off!
Have a look at the code editor to see a condition based on the user's input.
1. Click to see what the code does. Type "no" a few times. Then type
"yes".
2. Change the condition in the while statement so that the loop
repeats as long as response is NOT equal to "yes".
3. Click again and test it the same way as in step 1 - it should work the
same way.
Code explanation: Although both versions of this loop have the same
results for yes and no, in some cases there's a good reason to write the
condition one way as it can make a difference to other values.
Click here for an example.
22. Customising for loops
22.1. Use for loops for printing numbers
Let's take a closer look at the for loop: foriinrange(5):
print(i)
You can put any code inside the loop, not just print(i). To do this you just
indent any code that should be repeated - like if statements.
Let's try a few more examples to help show how it works.
1. Change the for loop on lines 2‑3 so that it prints out the numbers 0
- 8.
2. Write a for loop on lines 6‑8 that prints out "I will practice coding
every day!" 5 times.
3. Give yourself 3 cheers for learning for loops by putting 2 print
statements in the loop on lines 12‑13, one saying "Hip hip..." and
the second saying "Hooray!"
Click here for a more detailed look at the parts of a for loop.
22.2. Customising the start and end points
The syntax for a for loop statement is this:
foriinrange(start,stop,step):
Where:
start is the number you want the loop (or i) to start at,
stop is the number you want it to stop at, and
step is how much i should change by each time through the loop.
We will customize step in the next task!
1. Write a for loop on lines 2‑3 that prints the numbers from 1-10.
2. Write a for loop on lines 7‑8 that prints the numbers from 20-35.
3. Write a for loop on lines 12‑13 that prints the numbers from 9-18 in
the format "The next number is 9"- tip.
Note: The empty print() statements just put a gap between the output
from each loop so you can see where each starts and ends. Make sure you
don't remove them!
22.3. Customise the step
Now let's play with the step argument.
The step value lets us say how much i should change by on each iteration,
or pass through the loop.
We can use any positive OR negative number in step. For example:
foriinrange(0,20,2): counts up in 2s
foriinrange(5,35,5): counts up in 5s
foriinrange(21,0,‐3): counts DOWN in 3s:
Pretty simple but very useful (and fun)!
1. Click to see the initial code for Loop #1 in action.
2. Under the Loop #2 print statement, write a for loop that counts up
in 3s from 6 - 21.
3. Under Loop #3, write a for loop that counts down from 10 to 1
(just in 1s).
4. Under Loop #4, write a loop that counts down from 100 to 0in
10s.
Remember that it still won't reach the stop value, even if counting in a
different step.
22.4. Using the i variable
The i variable is just like any other variable. It stores an integer which can
be used the same way as any other integer variable - in calculations,
conditions, .format() and so on.
You can, in fact, call it any valid variable name too. You could write:
forspaghettiinrange(3):
print(spaghetti)
But it doesn't make as much sense!
Click here for some examples of when this IS useful.
1. Write a for loop statement on line 2 that will count from 1 to 12
inclusive.
2. Inside the loop, on line 3, write a print statement that uses i to
print out the 3 times table. You should print each line as "3 x 1 = 3"
and so on using .format().
3. Click to check that it works!
22.5. Review quiz!
Let's review for loops with a quiz.
Review Quiz Questions:
1. How many times will "Ni!" be printed out with the following loop?
2. What is the last number that the following loop will print out?
3. What is the 3
rd
number that the following loop will print out?
4. What is the output of the 4th time through this loop?
23. Refactoring turtle graphic loops
23.1. Refactor turtle code using loops
Let's play with some more turtle graphics. For loops work exactly the same
way with turtle graphics as they do with any other Python code.
In the editor you will see the code from lesson6 for drawing a square. It's
very repetitive so let's see if we can improve it.
1. Click to remind yourself of what it does.
2. Refactor the code using a for loop so that it is shorter.
3. Click to check that tiny still draws a square!
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23.2. Use a list to run a for loop
We saw that we can iterate (loop) through each letter in a word by using:
forletterin"Xylophone"
We can also make a list, and loop through each item in the list
automatically. A list looks like this: foods=["cereal","pizza",
"chocolate","cake"]
And a loop that runs using the list looks like this: forfoodinfoods:
print("{}isyum!".format(food))
1. In the editor a list called colors has been added. Add the colors
"blue" and "green" to the end of the list (in that order).
2. Refactor the for statement in the square code again so that it
loops through once for each color in colors.
3. On line 11 (before tiny.forward(150)) set pencolor() to the
current color.
23.3. Use loops to draw other shapes
Let's practice drawing some other shapes using for loops.
The #Gotoposition code has been used between exercises to make sure
the shapes don't overlap.
For all shapes, use .left() to turn, and 100 as the length of the sides.
1. Under the correct comment, set the pencolor() and draw a
hollow red hexagon using a for loop.
2. Under the correct comment, set the pencolor() and fillcolor()
and use a for loop to draw a triangle with a blue line and yellow
fill (tip).
3. Under the correct comment, draw a pink pentagon with pink fill.
Hint: If you're not sure how far to turn for the hexagon or pentagon this is
the rule:
"The external angles of polygons add up to 360".
So divide 360 by how many sides the shape has!
23.4. Spirographs!
Now let's have some fun!
Because we are using loops, we can do some cool things that would take us
a really long time to do if we had to write it all out line by line.
Let's have a go at drawing some spirograph-like designs.
1. Click to see what the code in the editor does.
2. Change the number 19 in the for loop to 100.
3. Change the angle inside .left() to a number close to 180.
4. Try adding in a second .forward() and a .left() or .right()
with a different distance and angle and after .left().
5. Experiment however you want!
6. Under #Changecolors add an if statement that checks if i==
20 and sets .color() to red if so.
7. Add elif branches that do the same for these: 40: orange, 60:
yellow, 80: green .
8. Finally, set the #Drawthespirograph code to the code in the
note below. Make sure it is indented so it is inside the loop and
click to finish the task.
Final movement code: spiro.forward(200)
spiro.left(184)
spiro.forward(40)
spiro.right(30)
23.5. Random numbers and RGB colors
Sometimes we want to pick a random number, and Python has a built in
random module for this purpose. random has tons of functions, but we're just
going to look at random numbers for now: importrandom#Haveto
importfirst
my_num=random.randrange(1,11)
#randrange()boundariesworkthesameasrange()
#Thischoosesanumberfrom1to10
In Python's turtle module, we can also set the color using numbers, by
giving it a number from 0-255 for red, green and blue: #RGBcolor
format:t.color((red,blue,green))
timmy.color((135,14,212))
Let's try out both of these with our spirograph!
1. In the editor is the code for a spirograph, without the code for
changing the color.
2. On line 10, create a variable called red and set it equal to a random
number between 0 and 255 inclusive.
3. On lines 11‑12, create two more variables called green and blue
and set them to a random number between 0 and 255 too.
4. On line 13, set spiro's color() to the random color using the red,
green,andblue variables.
5. Click several times to see the result.
Note on brackets - ().
24. Complex for loop example
24.1. Working through an example
Let's work through a problem that needs a for loop, in order to see how one
might be used in practice.
In this lesson, you will build a program that calculates the total and lowest
number of hours a user has spent online in the past week.
1. Firstly, let's ask the user for their name and store it in a variable
called name, on line 2.
2. Now on line 3, greet them with "Hello, ... !" using a print statement,
where ... is their name.
24.2. Create the for loop
Now we want to ask the user how many hours they have spent online each
day, for the past 7 days. We could write out 7 different input statements and
use 7 different variables, but that would take a long time!
The rule of 3 says we should use a loop if we're going to have to repeat
something 3 or more times.
1. On line 6, write a for loop statement that will repeat our code 7
times (we need i to go from 1 - 7).
2. Inside the loop, print i and click to check it works.
3. Delete this print statement and replace it with an input that asks
the user "How many hours did you spend online on day1? " and
stores it in a variable called hours. The next time through the loop
it should say "day2" and so on.
4. Underneath this, add print(hours) just to check it is storing the
numbers.
5. Click to check that the numbers you enter are printed correctly in
the console.
24.3. Adding up the total hours
The next step is to calculate the total number of hours they have entered.
1. On line 5, create a variable called total_hours and set it equal to
0.
2. Inside the loop, delete print(hours) and replace it with a
calculation that adds the number of hours that have been entered
to total_hours.
3. We need to use the input as a number, and they might say 3.5 or
similar, so we had better put float() around our input.
4. Print out totalhours AFTER the loop in the sentence "You spent
{} hours online in total".
5. Test your code and type in at least 1 float value.
6. (Optional) Format the float output in the final print statement to
1dp.
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24.4. Finding the lowest number
Finally, we are going to be really clever, and combine a selection structure
with our iterative or loop structure! We're going to use an if statement to
find the lowest number of hours.
This task uses a variable that stores a special value called None. This
basically creates an empty variable - we have given it a name, but there is
nothing stored inside it when we create it.
1. On line 6, add a variable called lowest_hours and set it equal to
None.
2. Inside your loop, write an if statement that will check if
lowest_hours equals None OR whether the number of hours
entered is less than lowest_hours. If you do not check these
conditions in this order your program will not work.
3. If this is true (inside the if), set lowest_hours equal to that
number of hours.
4. Add a print statement at the end of your program that says "The
least time you spent online in one day was {} hours"
5. Test your code!
6. (Optional) Format the float output to 1dp.
24.5. Review quiz!
Let's just double check that you understand for loops!
Review Quiz Questions:
1. What symbol goes at the end of a for loop statement to start a
block?
2. How many times does this loop repeat?
3. What is the first number this loop prints out?
4. What is the last number this loop prints out?
25. Introduction to while loops
25.1. Get to know the while loop
It's time to take a closer look at the while loop.
A while loop can be used to do the same things as a for loop, but it can
take a couple of extra lines of code: i=0
whilei<10:
print(i)
i+=1
It is important to understand how to run a while loop with a counter
though, so this lesson will let you practice that before we look at other uses
for a while loop!
Click here for an explanation of the code above.
1. Change the while loop on lines 2‑5 so that it prints out the
numbers 0 - 8.
2. Write a while loop starting on line 8 that prints out "I will practice
coding every day!" 5 times.
3. Give yourself 3 cheers for learning while loops by putting 2 print
statements in the loop on lines 17‑18, one saying "Hip hip..." and
the second saying "Hooray!"
4. Add 1 to i after the print statements so that you don't get an
infinite loop!
25.2. Customise the start and stop points for a while loop
Just like a for loop, we can also customise the start and stop points for a
while loop. For example, a loop that starts at 5 and stops at 15: i=5
#Thisisourstartpoint
whilei<=15:#Thisisourendpoint
print(i)
i+=1#Don'tforgetthis!
Because we can write any kind of condition we want in a while loop, we
can use <= in this code and write 15, instead of in a for loop where we have
to write one number higher than the last one we want printed.
1. Write a while loop under the relevant comment that prints the
numbers from 1-5.
2. Write a while loop under the relevant comment that prints the
numbers from 10-25.
3. Combine the variable i with a print statement using .format() by
writing a while loop under the relevant comment that prints the
numbers from 9 to 18 in the format: "The next number is 9".
Note: It also works perfectly fine if you write the example loop this way,
using <16: i=5#Thisisourstartpoint
whilei<16:#Thisisourendpoint
print(i)
i+=1#Don'tforgetthis!
25.3. Customise the step in a while loop
Now let's play with the step in a while loop!
Remember that the step is how much i should change by on each
iteration, or pass through the loop.
To change the step in a while loop, we simply change the line: i+=1
We can change it to pretty much any kind of calculation that changes i:
i‐=3
i*=2
i=i*3‐2
These will all give quite different results. We can also use any of these in our
conditions: <,>,<=,>=,==,!=,and,or,not
1. Click to see the initial code for Loop #1 in action.
2. Under the Loop #2 print statement, write a while loop that counts
up in 3s from 6 - 21.
3. Write a while loop that counts down from 10 to 1 (just in 1s).
4. Write a while loop that counts down from 100 to 0in 10s.
Note: If you use < rather than <= it will not print the stop value, even if
counting in a different step.
25.4. Using the i variable
Just like in a for loop, the i variable can be used in calculations, substituted
into a string using .format() and so on inside a while loop.
You can also call it any valid variable name, but again it's best to use i if it's
just a simple counter, or choose something that makes sense rather than:
peanut_butter=0
whilepeanut_butter<=10:
print(peanut_butter)
Click here for some examples of when this IS useful.
1. Create a variable i and write a while loop statement on lines 2‑3
that will count from 1 to 12 inclusive.
2. Inside the loop, write a print statement that uses i to print out the
3 times table. You should print each line as "3 x 1 = 3" and so on
using .format().
3. Don't forget to increment i!
4. Click to check that it works!
25.5. Review quiz
Now let's review while loops with a quiz!
Review Quiz Questions:
1. How many times will "Hello!" be printed out with the following
loop?
2. What is the last number that the following loop will print out?
3. What is the 3
rd
number that the following loop will print out?
4. What is the highest number that the following loop will print out?
26. Alternate loop counters
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26.1. Run a while loop with a variable other than i
Sometimes we might want to run a loop that depends on a variable that has
a purpose in our code, rather than just a generic counter like i. For example,
if we're giving a user 3 tries to guess a password: num_tries=0
whilenum_tries<3:
password=input("Password:")
ifpassword=="ekki‐ekki‐ekki":
print("Correct!")
break
else:
print("Incorrect.")
num_tries+=1
In this case, rather than i it makes sense to have a variable name that shows
what the loop is doing. This is part of writing well-documented code.
Let's try an example.
1. Write a while loop statement on line 8 that will make the loop run
as long as the user has lives left.
2. Inside the loop, ask the user to guess the number you're thinking of
and store it as guess.
3. Write an if statement that checks if the user's guess is correct and
says "Correct!" if so.
4. Add an else that prints "Wrong!" if the user is incorrect and
removes one life then your code.
5. (Optional) Set lives to 0 if they get it correct, so that the loop will
stop running.
Note that the break keyword is used to exit the loop early. We'll cover this in
the next lesson.
26.2. Run a while loop with user input
We might want our loop to depend on what a user types in. The example
code shows one way we could do this.
In this example, the user's input of a number is used to change the
variable total_servings which is running the loop. Once the user's input
adds up to 5 servings, the loop will stop.
A while loop is useful here; we don't want it to run 5 times because the user
might type 5 as their first input!
1. Read the code in the code editor.
2. On line 5, write a while loop statement that will repeat until the
user has no pocket money left.
3. Inside the loop, ask the user for the price of the item they want to
buy and store it as price.
4. Subtract the price from their pocket money and then tell them "You
have ${} left". Don't forget to format the float output as money!
We will improve on the logic of this in the next couple of tasks, because right
now they can spend as much as they want the last time and end up in debt!
26.3. Use a Boolean variable to run a while loop
We don't just have to use greater thans and less thans to run our loops; we
can also use Boolean variables.
Instead of storing the result of a comparison like this:
my_var=3<=5#my_varisTrue
We can store either True or False straight into a variable:
repeat=True
We can then run a loop using : whilerepeat==True:#Aslongas
repeatisTrue
#Dosomething
We can then "switch off" the variable somewhere inside the loop by setting it
to False.
1. Create a variable on line 3 called still_shopping and set it to
True.
2. Write a condition in the while statement on line 6 that will keep
our loop going as long as the user is still_shopping.
3. On line 12, ask the user if they would like to keep shopping and
store the response in a variable called confirm.
4. Below this, write an if statement that sets still_shopping to
False if the user says "no".
5. Click and test your program with both "yes" and "no" to keep
shopping.
26.4. Use more complex conditions to run a while loop
Uh oh, now our shopper can spend as much as they want and get into as
much debt as they like! We need to fix this.
In task2 we stopped the loop when the user ran out of money, and in task3
we stopped it when they said they were done. Now we'll combine the two
so that the program makes more sense.
Are you ready?
1. Using the and operator, make the loop run as long as the user is
still_shopping AND they still have pocket money left.
2. Now let's stop them getting into debt! On line 10, write an if
statement that checks whether their pocket money minus the
price they entered is $0 or more.
3. Indent the calculation on line 11 in so that it only happens if they
can afford the item.
4. Add an else branch that prints "You can't afford that".
5. Click and test your code with all the tests listed in the note below.
6. (Optional) You might like to put the confirm segment of code
inside its own if statement so that it will only be run if the user still
has money.
Tests:
1) Using up exactly all of your money.
2) Spending some and then saying "no" to keep shopping.
3) Trying to spend more than you have.
26.5. Use conditions to make sure input is within the right boundaries
Another good use for this kind of loop is to force the user to enter valid
input. Let's look first at how to get the user to enter a number within a
certain range.
Say we want the user's age. A sensible range for this might be 0 - 100. We
don't want them to enter a negative number, or tell us they're 225.
We can use our loop conditions to do this, by making the loop repeat if they
enter either of these values: age=int(input("Howoldareyou?"))
whileage<0orage>100:
print("Pleaseenteravalidage.")
age=int(input("Howoldareyou?"))
This loop will repeat until they enter a valid number between 0 and 100.
Here's an example for you to try:
1. Ask the user how many minutes of exercise they did today and
store the response in a variable called time. Don't forget int()!
2. The number of minutes in a day is 1440. Write a while loop that
will continue until they enter a value from 0 to 1440.
3. Inside the loop print "Please enter a valid number. The total
minutes in one day is 1440".
4. Prompt them again for their exercise time.
27. Using break and continue
27.1. Use the break and continue statements with for loops
In the last lesson we saw the word break. Break is used to exit a loop early,
usually if a condition is met: foriinrange(10):
print(i)
ifi==8:
break#Exittheloopafter8hasbeenprinted
This break statement should be used sparingly because if it's not used well
it can make your code really messy and hard to debug.
We also have continue which will skip any lines of code after it in the loop
for that round or pass, and start again at the first line of code in the loop for
the next iteration.
1. Click to see what the code in the code editor does - look closely at
the output.
2. Change the loop on lines 2‑5 so that it stops running after the
number 13 is printed.
3. Change the loop on lines 10‑13 so that it skips printing the number
9.
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27.2. Using an else with a for loop
Python has a cool feature where you can add an else clause to a for loop!
That might sound weird, but you'll see what it does if you the example code.
The else branch runs only if the loop is NOT broken, in this example, if the
thing we're looking for is not found.
This gives us a way to make our Knights of Ni online training password
program much more logical.
1. Complete the for statement on line 4 so that the loop will run 3
times.
2. If the user guesses the password correctly, print "Correct,
authorisation complete."
3. After this line add a break statement to exit the loop since they
were right (still inside the if branch).
4. The first else branch goes with the if and runs if the guess is
incorrect, so print "Incorrect" in here.
5. The second else on line 12 runs if the loop exits normally without
being broken, in this case if the user fails 3 times. Print ""Sorry, 3
incorrect guesses, your account has been locked." in this branch.
6. Click and test your code!
27.3. Use break with a while loop
Because a while loop checks a condition, we can usually just change a
variable inside the loop to switch it off.
Sometimes we might want to do something small, however, that we don't
want to make a whole new variable for. In this case we might use break with
an infinite loop.
An infinite loop can be done in many ways (often accidentally!) but a
deliberate one might look like this: whileTrue:
#dosomething
We can't ever switch it off because the only part to our condition is the word
True, which is always True!
Let's look at forcing a user to enter valid input (validation).
1. Complete the while statement so it is an infinite loop (line 2).
2. If they type in "true" or "false", break the loop (line 6).
3. Otherwise print "Please answer with true or false" (line 8).
4. Click and test your code several times to check all of the branches.
Note that this code is just a snippet; it doesn't actually check if they are right
or wrong, just that they have entered a valid answer. This is important for
writing really good code!
27.4. Use try/except to force numerical input
At the moment, when we use: int(input("Enteranumber:")) our
program crashes if we type in letters, or leave it blank - you can test this by
running the example code.
To get around this, we use try and except. It looks like this: whileTrue:
try:
number=int(input("Enteranumber:"))
break
exceptValueError:
print("Pleaseenteranumber!")
Click here for an explanation.
1. Create an infinite loop using the while statement on line 2.
2. This time we're expecting a height such as 1.8, so add float() to
the input on line 4.
3. If the user enters invalid data (in the except branch), print "Please
enter a valid height in metres e.g. 1.65".
4. Click and test the program with a word, an integer and a float.
Note: If you think the input might be an integer or a float, then use float().
If you don't want to accept decimals, then use int().
27.5. Review quiz time!
Let's review break and continue.
Review Quiz Questions:
1. What does the break statement do?
2. What does the continue statement do?
3. What does the following code print out?
4. What are the keywords that can be used to write code that will
force a user to enter a number?
28. Testing for infinite loops
28.1. Testing expected values
In this lesson you will learn about testing your code. It's really important to
know how to test your code thoroughly if you want to write good code that
works.
There are generally 3 types of input values that we test. For example, if
we're asking the user for the number of hours they exercised for today:
Expected values - the values you expect your user to enter. In this case
0-24 inclusive (24 hours in a day).
Boundary values - data that is on either side of the acceptable limits. In
this case -1 and 0 and 24 and 25.
Invalid or exceptional values - values your program should NOT
accept. In this case for example: three, -3, lots.
1. Click twice, and test this code with an expected value.
2. Click again and try typing in 25.
3. Click once more and try typing in three.
This code works fine for expected values, but it crashes on invalid values
containing letters, and should make us try again if we type in an outer
boundary like 25 or an invalid number like -3.
Extra Note.
28.2. Test boundary values
Here is a new version of our code that has been set up to handle the
boundary input cases properly.
This code snippet now only accepts numbers between 0 and 24. Let's test it
out!
1. Click and type in the first boundary value -1, then the value 0.
2. Click and type in the first boundary value 25, then the value 24.
3. Click one more time and enter an invalid value that contains
letters.
Usually you would pick a sample of expected and invalid values to test your
program on, but it's best to test ALL boundary values because it's the only
way to be sure that you've got all your conditions right.
28.3. Test invalid or exceptional values
Now we just need to make it so that our program doesn't crash when an
invalid value that contains text is entered!
Remember try/except? We're going to use that here.
This version of our code uses the whileTrue: loop to force valid input. If
the user enters a number, we will try to convert it using float(), and if
that works then we will check if their number is in the right range. If not, we
ask them to enter a number in the right range and stay in the loop.
Otherwise (else), if they have entered a number in the right range, we will
break.
The except block runs if the number can't be converted using float(), and
gives them a specific error message. This code handles all expected,
boundary and invalid values.
1. Replace the ??? to complete the if statement on line 5 with a
condition that will make sure the user's input is within the
boundary values.
2. Replace the ??? on line 9 with the correct keyword.
3. Click and test the program with a boundary value that is outside
the boundary then with an expected value.
4. Click and test the program with an invalid value that contains
letters then with an expected value.
Don't worry if the code is a little bit confusing, this is getting quite complex!
If you are keeping up, well done.
28.4. Other ways of testing input data
Python has some other built-in functions that we can use to find out
information about strings: .isnumeric()#Trueifastringcontains
onlynumbers
.isalpha()#Trueifastringhasonlyletters
.isalnum()#Trueifastringhasonlylettersand/ornumbers
(alphanumeric)
An example of where this might be useful is if you wanted to force a user to
enter text only e.g. a name, or numbers and letters only e.g. a serial
number.
1. The code in the editor forces the user to enter a name with no
numbers in it. Click and test it with "9000" to see what happens.
2. Click again and test it with "R2D2".
3. Click once more and enter a valid name.
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28.5. Review quiz
Let's review what we've covered about testing.
Review Quiz Questions:
1. Which of the following is (only) an expected value for this code?
2. Which of the following is an acceptable boundary value for this
code?
3. Which of the following is an unacceptable boundary value for this
code?
4. Which of the following is an invalid value for this code?
29. Debugging loops
29.1. Bug hunt #3
It's time to practice finding bugs in loops!
Many of the bugs you'll come across are similar to those that are common in
sequential and conditional code - spelling, punctuation, indenting,
getting the conditions right and so on.
Most bugs don't cause a program to crash but they make it do the wrong
thing. Read the comments in the code carefully to help you find all of the
bugs.
1. Find the bug in the for loop on lines 2‑3.
2. Find the 2 bugs in the while loop on lines 6‑9.
29.2. Laying out code correctly
It's important to lay out your code well with proper punctuation, brackets,
operators and white space.
1. Fix the bug on line 5.
2. Fix the bug on line 9.
3. Some of the indenting is wrong. This code should be a for/else
loop with an if/else inside it. Fix up the indenting so that it works
properly.
4. Click and test it to make sure it works correctly.
29.3. Debug while loop conditions
It's very easy to make a mistake with the condition in a while loop,
especially with the boundary values.
1. Fix the bug on line 2.
2. Click and test the boundary values for the loop.
3. Fix the condition on line 4 so that the loop only accepts valid values.
4. Which value was being accepted when it shouldn't have been?
29.4. Debugging unintentional infinite loops or loops that don't run
Sometimes we use an infinite loop for a reason, but often they can happen
accidentally because we mess up our code.
This interpreter will only print out so many lines of an infinite loop, but in a
normal program an infinite loop would cause the program to crash or
freeze!
1. Fix the bug that is making the first loop infinite!
2. Fix the bug that makes the second loop not even run at all.
29.5. Debugging intentionally infinite loops!
Now, if we actually want a loop to be infinite and repeat forever until we get
the data or result we are looking for, then there's something really important
we have to remember to do when we find it!
1. Click and test this with some invalid numbers or words.
2. Click and type in an expected value. Hmm...
3. Fix the bug that is making this loop repeat even with valid input.
30. Review lessons 21-29
30.1. Review loops
Time to review lessons21-29 and make sure you understand iteration, or
loops!
Loops are also called iterative programming structures because they make
code iterate or repeat. Each time that you go through a loop it is called an
iteration or sometimes a pass.
Click here for a for loop summary.
Click here for a while loop summary.
1. Write a for loop that prints the numbers from 2 - 18.
2. Write a while loop that does the same.
30.2. Loops review quiz
Let's mix it up a bit with a quiz!
Review Quiz Questions:
1. Which for loop statement would accurately print out the numbers
3 - 7?
2. Which for loop statement would accurately print out "Hello" 7
times?
3. What is used inside a while loop to make it count up in 2s?
4. When is a for loop usually used?
5. When is a while loop usually used?
30.3. Review using the i variable
We can use i (or whatever we have called a variable that runs our loop)
inside the loop just like a regular variable.
It's important to know what you want to use i for, though, so you can write
your loop appropriately.
1. Click and look at the code and the output.
2. Fix the range() in the first loop so that it prints out "day1, day2"
etc.
3. The for loops on lines 7‑9 should print out the 1-3 times tables
starting with "1 x 1 = 1" and ending with "3 x 3 = 9". Fix up the
range() in each so it works properly.
Click here for explanation of the times tables loops.
30.4. Another review quiz!
How much do you remember?
Review Quiz Questions:
1. Which is the best description of this for loop?
2. What is a Boolean variable?
3. What does this code draw?
4. What does this code draw?
30.5. Review writing for and while loops that depend on user input
In this final task you have one more chance to write some loops that are
affected by user input.
1. Under the #Forloop comment, ask the user to enter a number
and store it as maximum.
2. Write a for loop that will print out all the numbers from 0 up to but
not including the user's chosen number.
3. Under the #Whileloop comment, ask the user "What is the coolest
programming language?" and store their input as answer.
4. Write a while loop that will force them to keep answering until their
answer is "python".
5. (Optional) Change your while loop so that it will check if their
answer contains the word python, so that answers like "Python for
sure!" and "it's python!" are accepted (hint: you might find
keywords like and,or,not,in etc. useful here).
3/3/2017
22/22