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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:
            say = 'FizzBuzz!'
        elif number % 5 == 0:
            say = 'Buzz!'
        elif number % 3 == 0:
            say = 'Fizz!'
        else:
            say = str(number)
        
        return say

>>> classic_fizzbuzz(15)
'FizzBuzz!'

>>> classic_fizzbuzz(13)
'13'

As an alternative, the example above can be re-written to only use if statements with returns. However, re-writing in this way might obscure that the conditions are intended to be mutually exclusive and could lead to future bugs or maintenance issues.

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

>>> classic_fizzbuzz(15)
'FizzBuzz!'

>>> classic_fizzbuzz(13)
'13'

Conditionals can also be nested.

>>> def driving_status(driver_age, test_score):
        if test_score >= 80: 
            if 18 > driver_age >= 16:
                status = "Student driver, needs supervision."
            elif driver_age == 18:
                status = "Permitted driver, on probation."
            elif driver_age > 18:
                status = "Fully licensed driver."
        else:
             status = "Unlicensed!"
        
        return status


>>> driving_status(63, 78)
'Unlicensed!'

>>> driving_status(16, 81)
'Student driver, needs supervision.'

>>> driving_status(23, 80)
'Fully licensed driver.'

Conditional expressions or “ternary operators”

While Python has no specific ? ternary operator, it is possible to write single-line conditional expressions. These take the form of <value if True> if <conditional test> else <value if False>. Since these expressions can become hard to read, it’s recommended to use this single-line form only if it shortens code and helps readability.

def just_the_buzz(number):
    return 'Buzz!' if number % 5 == 0 else str(number)
    
>>> just_the_buzz(15)
'Buzz!'

>>> just_the_buzz(7)
'7'

Truthy and Falsy

In Python, any object can be tested for truth value, and can therefore be used with a conditional, comparison, or boolean operation. Objects that are evaluated in this fashion are considered “truthy” or “falsy”, and used in a boolean context.

>>> def truthy_test(thing):
        if thing:
            print('This is Truthy.')
        else:
            print("Nope. It's Falsey.")


# Empty container objects are considered Falsey.
>>> truthy_test([])
Nope. It's Falsey.

>>> truthy_test(['bear', 'pig', 'giraffe'])
This is Truthy.

# Empty strings are considered Falsey.
>>> truthy_test('')
Nope. It's Falsey.

>>> truthy_test('yes')
This is Truthy.

# 0 is also considered Falsey.
>>> truthy_test(0)
Nope. It's Falsey.

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Originally from Exercism python concepts