If Statements

Programming often involves examining a set of conditions and deciding which action to take based on those conditions. Python's if statement allows you to examine the current state of a program and respond appropriately to that state.

A Simple Example

# Conditional formatting based on car type
cars = ['audi', 'bmw', 'subaru', 'toyota']

for car in cars:
    if car == 'bmw':
        print(car.upper())
    else:
        print(car.title())

Output:

Audi
BMW
Subaru
Toyota

Conditional Tests

At the heart of every if statement is an expression that can be evaluated as True or False and is called a conditional test.

Checking for Equality

# Test for equality
car = 'bmw'
print(car == 'bmw')  # True

car = 'audi'
print(car == 'bmw')  # False

Case Sensitivity in Tests

Testing for equality is case sensitive in Python:

# Case-sensitive comparison
car = 'Bmw'
print(car == 'bmw')  # False

# Convert to lowercase for comparison
car = 'Bmw'
print(car.lower() == 'bmw')  # True
print(car)  # Original value unchanged: 'Bmw'

Checking for Inequality

# Test for inequality
requested_topping = 'mushrooms'
if requested_topping != 'anchovies':
    print("Hold the anchovies!")

Output:

Hold the anchovies!

Numerical Comparisons

# Numerical comparisons
answer = 17
if answer != 42:
    print("That is not the correct answer.")

a = 0
b = 10
print(a > b)   # False
print(b >= a)  # True

You can use these comparison operators:

• == (equal to)
• != (not equal to)
• > (greater than)
• >= (greater than or equal to)
• < (less than)
• <= (less than or equal to)

Checking Multiple Conditions

Using and to Check Multiple Conditions

# Both conditions must be True
a = 0
b = 10
print(a > 1 and b > 5)  # False (first condition is False)

Using or to Check Multiple Conditions

# At least one condition must be True
a = 0
b = 10
print(a > 1 or b > 5)   # True (second condition is True)

Checking Whether a Value is in a List

# Check if value is in list
animals = ['cat', 'dog', 'pig']
print('cat' in animals)     # True
print('tiger' in animals)   # False

Checking Whether a Value is Not in a List

# Check if value is not in list
animals = ['cat', 'dog', 'pig']
print('cat' not in animals)     # False
print('tiger' not in animals)   # True

Boolean Expressions

A Boolean expression is just another name for a conditional test:

# Boolean values
a = True
b = False
print(a, b)  # True False

If Statements

Simple if Statements

The simplest kind of if statement has one test and one action:

# Basic if statement
a = 10
if a > 0:
    print('Yes, a > 0.')

Output:

Yes, a > 0.

if-else Statements

An if-else block is similar to a simple if statement, but the else statement allows you to define an action that should be taken when the conditional test fails:

# if-else statement
a = 10
if a > 0:
    print('Yes')
else:
    print('No')

Output:

Yes

if-elif-else Chain

Often, you'll need to test more than two possible situations. Python's if-elif-else chain makes this possible:

# if-elif-else chain
age = 12
if age < 4:
    print("$0")
elif age < 18:
    print("$25")
else:
    print("$45")

Output:

$25

You can also store the result in a variable:

# Store result in variable
age = 12
if age < 4:
    price = 0
elif age < 18:
    price = 25
else:
    price = 45
    
print(f"Your cost is ${price}")

Output:

Your cost is $25

Using Multiple elif Blocks

You can use as many elif blocks as you need:

# Multiple elif blocks
age = 12
if age < 4:
    price = 0
elif age < 18:
    price = 25
elif age < 65:
    price = 40
else:
    price = 20
    
print(f"Your admission cost is ${price}.")

Omitting the else Block

Python does not require an else block at the end of an if-elif chain. The else block is a catchall statement that matches any condition that wasn't matched by a specific if or elif test.

Using if Statements with Lists

Checking for special values in a list and handling them appropriately is one task you'll do again and again when you're working with lists:

# Handling special cases in a list
requested_toppings = ['mushrooms', 'green peppers', 'extra cheese']

for requested_topping in requested_toppings:
    if requested_topping == 'green peppers':
        print("Sorry, we are out of green peppers right now.")
    else:
        print(f"Adding {requested_topping}.")

print("\nFinished making your pizza!")

Output:

Adding mushrooms.
Sorry, we are out of green peppers right now.
Adding extra cheese.

Finished making your pizza!

Checking That a List is Not Empty

# Check for empty list
requested_toppings = []

if requested_toppings:
    for requested_topping in requested_toppings:
        print(f"Adding {requested_topping}.")
    print("\nFinished making your pizza!")
else:
    print("Are you sure you want a plain pizza?")

Using Multiple Lists

# Working with multiple lists
available_toppings = ['mushrooms', 'olives', 'green peppers',
                     'pepperoni', 'pineapple', 'extra cheese']
requested_toppings = ['mushrooms', 'french fries', 'extra cheese']

for requested_topping in requested_toppings:
    if requested_topping in available_toppings:
        print(f"Adding {requested_topping}.")
    else:
        print(f"Sorry, we don't have {requested_topping}.")

print("\nFinished making your pizza!")

Styling Your if Statements

The PEP 8 style guide recommends the following approach for styling conditional tests:

• Use a single space around comparison operators: ==, >=, <=
• Don't put spaces around the inequality operators: !=

For example:

# Good style
if age < 4:
    
# Avoid
if age<4:

Summary

In this chapter you learned how to:

• Write conditional tests using comparison operators
• Combine conditional tests using and and or
• Check whether values are or aren't in lists
• Write simple if statements, if-else blocks, and if-elif-else chains
• Use conditional tests to handle special situations in lists
• Style your conditional tests according to PEP 8 guidelines

Conditional tests form the foundation of decision-making in programming, allowing your programs to respond appropriately to different situations.