17. Selection

if, elif and else provide choices or branches in the code.
They all are used in lines of code which end with a colon, :.
Both if and elif test a condition that returns True or False. Their indented code block runs if the condition is True.
Multiple elif can be used to provide more choices.
The else block does not have a condition.
The else block only runs if all the previous conditions were False.

17.1. if

../_images/if.png
if requires a condition that returns True or False.
x = 10
if x > 0:
    x = x - 1
    print(x)

17.2. if - else

../_images/if_else.png
The else block does not have a condition.
The else block only runs if all the previous conditions were False.
x = 10
if x > 0:
    x = x - 1
    print(x)
else:
    print("blastoff!")

17.3. if - elif

../_images/if_elif.png
elif can be used to provide another choice by testing to see if its condition is True.
x = 10
if x > 0:
    x = x - 1
    print(x)
elif x == 0:
    print("blastoff!")
    x = 10

17.4. If - elif - else

../_images/if_elif_else.png
Using if, elif and else together provides 3 branches in the code.
x = 10
if x > 0:
    x = x - 1
    print(x)
elif x == 0:
    print("blastoff!")
else:
    x = 10

17.5. If - elif - elif - else

../_images/if_elif_elif_else.png
Using if, two elif and else together provides 4 branches in the code.
The logical keyword and requires both conditions to be True for the combined condition to be True.
x = 10
y = 10
if x > 0 and y > 0:
    x = x - 1
    print(x)
elif x == 0:
    print("blastoff!")
elif x == 0:
    print("blastoff!")
else:
    x = 10