python/concepts/conditionals/introduction.md

Introduction

In Python, if, elif (a contraction of 'else and if') and else statements are used to control the flow of execution and make decisions in a program. Unlike many other programming languages, Python versions 3.9 and below do not offer a formal case-switch statement, instead using multiple elif statements to serve a similar purpose.

Python 3.10 introduces a variant case-switch statement called pattern matching, which will be covered separately in another concept.

Conditional statements use expressions that must resolve to True or False -- either by returning a bool directly, or by evaluating "truthy" or "falsy".

x = 10
y = 5

# The comparison '>' returns the bool 'True',
# so the statement is printed.
if x > y:
    print("x is greater than y")
...
>>> x is greater than y

When paired with if, an optional else code block will execute when the original if condition evaluates to False:

x = 5
y = 10

# The comparison '>' here returns the bool False,
# so the 'else' block is executed instead of the 'if' block.
if x > y:
    print("x is greater than y")
else:
    print("y is greater than x")
...
>>> y is greater than x

elif allows for multiple evaluations/branches.

x = 5
y = 10
z = 20

# The elif statement allows for the checking of more conditions.
if x > y > z:

    print("x is greater than y and z")
elif y > x > z:

    print("y is greater than x and z")
else:
    print("z is greater than x and y")
...
>>> z is greater than x and y

Boolean operations and comparisons can be combined with conditionals for more complex testing:

>>> def classic_fizzbuzz(number):
        if number % 3 == 0 and number % 5 == 0:
            return 'FizzBuzz!'
        elif number % 5 == 0:
            return 'Buzz!'
        elif number % 3 == 0:
            return 'Fizz!'
        else:
            return str(number)

>>> classic_fizzbuzz(15)
'FizzBuzz!'

>>> classic_fizzbuzz(13)
'13'