2-if-statements.md

if statements

In Python we can control the execution of code based on conditions with if statements.

if statements

Using if, we can execute part of a program conditional on some statement being true.

traffic_light = 'green'
if traffic_light == 'green':
    print("move car!")

Note on indentation

In Python, blocks of code are defined using indentation.

This means that everything indented after an if statement is only executed if the statement is True.

If the statement is False, the program skips all indented code and resumes at the first line of unindented code.

statement = False
if statement:
    # if statement is True, then all code here
    # gets executed but not if statement is False
    print("The statement is true")
    print("Else, this would not be printed")
# the next lines get executed either way
print("Hello, world,")
print("Bye, world!")

if-else statements

We can add more conditions to the if statement using else and elif (short for else if):

traffic_light = 'red'
if traffic_light == 'green':
    print("drive")
elif traffic_light == 'orange':
    # executed if conditional statement is True and previous [el]if statements
    # are false
    print("better drive faster")
else:
    # executed if all other [el]if statements return False
    print("maybe stop")

What is True?

Non-boolean values can be used in an if statement. It is important to know which values are considered True versus False.

if "am I True?":
    print("Yes! :)")
else:
    print("No :(")

Non-empty strings are considered True while empty strings are False.

In Python the following values are interpreted as False:

• False
• None
• numeric zero of all numeric types
• empty strings
• empty containers (including strings, tuples, lists, dictionaries, sets and frozensets)

All other values are interpreted as True.