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Beginner's Python

Cheat Sheet –

If Statements

and While Loops

Conditional Tests

A conditional test is an expression that can be evaluated as

True or False. Python uses the values True and False to

decide whether the code in an if statement should be

executed.

Checking for equality

A single equal sign assigns a value to a variable. A double equal

sign (==) checks whether two values are equal.

>>> car = 'bmw'

>>> car == 'bmw'

True

>>> car = 'audi'

>>> car == 'bmw'

False

Ignoring case when making a comparison

>>> car = 'Audi'

>>> car.lower() == 'audi'

True

Checking for inequality

>>> topping = 'mushrooms'

>>> topping != 'anchovies'

True

Numerical comparisons

Testing numerical values is similar to testing string values.

Testing equality and inequality

>>> age = 18

>>> age == 18

True

>>> age != 18

False

Comparison operators

>>> age = 19

>>> age < 21

True

>>> age <= 21

True

>>> age > 21

False

>>> age >= 21

False

Python Crash Course

A Hands-On, Project-Based

Introduction to Programming

nostarch.com/pythoncrashcourse2e

If statements

Several kinds of if statements exist. Your choice of which

to use depends on the number of conditions you need to

test. You can have as many elif blocks as you need, and

the else block is always optional.

Simple if statement

age = 19

if age >= 18:

print("You're old enough to vote!")

If-else statements

age = 17

if age >= 18:

print("You're old enough to vote!")

else:

print("You can't vote yet.")

The if-elif-else chain

age = 12

if age < 4:

price = 0

elif age < 18:

price = 5

else:

price = 10

print(f"Your cost is ${price}.")

What are if statements? What are while loops?

If statements allow you to examine the current state

of a program and respond appropriately to that state.

You can write a simple if statement that checks one

condition, or you can create a complex series of if

statements that identify the exact conditions you're

looking for.

While loops run as long as certain conditions remain

true. You can use while loops to let your programs

run as long as your users want them to.

Checking multiple conditions

You can check multiple conditions at the same time. The

and operator returns True if all the conditions listed are

True. The or operator returns True if any condition is True.

Using and to check multiple conditions

>>> age_0 = 22

>>> age_1 = 18

>>> age_0 >= 21 and age_1 >= 21

False

>>> age_1 = 23

>>> age_0 >= 21 and age_1 >= 21

True

Using or to check multiple conditions

>>> age_0 = 22

>>> age_1 = 18

>>> age_0 >= 21 or age_1 >= 21

True

>>> age_0 = 18

>>> age_0 >= 21 or age_1 >= 21

False

Boolean values

A boolean value is either True or False. Variables with

boolean values are often used to keep track of certain

conditions within a program.

Simple boolean values

game_active = True

can_edit = False

Conditional tests with lists

You can easily test whether a certain value is in a list. You

can also test whether a list is empty before trying to loop

through the list.

Testing if a value is in a list

>>> players = ['al', 'bea', 'cyn', 'dale']

>>> 'al' in players

True

>>> 'eric' in players

False

Conditional tests with lists (cont.)

Testing if a value is not in a list

banned_users = ['ann', 'chad', 'dee']

user = 'erin'

if user not in banned_users:

print("You can play!")

Checking if a list is empty

players = []

if players:

for player in players:

print(f"Player: {player.title()}")

else:

print("We have no players yet!")

While loops (cont.)

Letting the user choose when to quit

prompt = "\nTell me something, and I'll "

prompt += "repeat it back to you."

prompt += "\nEnter 'quit' to end the program. "

message = ""

while message != 'quit':

message = input(prompt)

if message != 'quit':

print(message)

Using a flag

prompt = "\nTell me something, and I'll "

prompt += "repeat it back to you."

prompt += "\nEnter 'quit' to end the program. "

active = True

while active:

message = input(prompt)

if message == 'quit':

active = False

else:

print(message)

Using break to exit a loop

prompt = "\nWhat cities have you visited?"

prompt += "\nEnter 'quit' when you're done. "

while True:

city = input(prompt)

if city == 'quit':

break

else:

print(f"I've been to {city}!")

More cheat sheets available at

ehmatthes.github.io/pcc_2e/

While loops (cont.)

Using continue in a loop

banned_users = ['eve', 'fred', 'gary', 'helen']

prompt = "\nAdd a player to your team."

prompt += "\nEnter 'quit' when you're done. "

players = []

while True:

player = input(prompt)

if player == 'quit':

break

elif player in banned_users:

print(f"{player} is banned!")

continue

else:

players.append(player)

print("\nYour team:")

for player in players:

print(player)

Accepting input

You can allow your users to enter input using the input()

statement. All input is initially stored as a string.

If you want to accept numerical input, you'll need to

convert the input string value to a numerical type.

Simple input

name = input("What's your name? ")

print(f"Hello, {name}.")

Accepting numerical input using int()

age = input("How old are you? ")

age = int(age)

if age >= 18:

print("\nYou can vote!")

else:

print("\nYou can't vote yet.")

Accepting numerical input using float()

tip = input("How much do you want to tip? ")

tip = float(tip)

While loops

A while loop repeats a block of code as long as a condition

is True.

Counting to 5

current_number = 1

while current_number <= 5:

print(current_number)

current_number += 1

Accepting input with Sublime Text

Sublime Text doesn't run programs that prompt the user for

input. You can use Sublime Text to write programs that

prompt for input, but you'll need to run these programs from

a terminal.

Avoiding infinite loops

Every while loop needs a way to stop running so it won't

continue to run forever. If there's no way for the condition to

become False, the loop will never stop running. You can

usually press Ctrl-C to stop an infinite loop.

An infinite loop

while True:

name = input("\nWho are you? ")

print(f"Nice to meet you, {name}!")

Removing all instances of a value from a list

The remove() method removes a specific value from a list,

but it only removes the first instance of the value you

provide. You can use a while loop to remove all instances

of a particular value.

Removing all cats from a list of pets

pets = ['dog', 'cat', 'dog', 'fish', 'cat',

'rabbit', 'cat']

print(pets)

while 'cat' in pets:

pets.remove('cat')

print(pets)

Breaking out of loops

You can use the break statement and the continue

statement with any of Python's loops. For example you can

use break to quit a for loop that's working through a list or a

dictionary. You can use continue to skip over certain items

when looping through a list or dictionary as well.