Booleans and truthiness

The bool type has exactly two values: True and False (capital first letter, no quotes). Every comparison produces one of them.

is_raining = True
has_umbrella = False
print(is_raining, type(is_raining))
print(5 > 3)

Booleans are numbers in disguise

True behaves like 1 and False like 0. That makes counting easy:

print(True + True + False)

answers = [True, False, True, True]
print("Correct answers:", sum(answers))

Truthy and falsy

In an if or while, Python accepts any value, not just booleans. It treats some values as false (falsy) and everything else as true (truthy).

Falsy values:

• False and None
• zero: 0, 0.0
• empty collections: "", [], (), {}, set()

name = ""
if name:
    print("Hello,", name)
else:
    print("No name given")

cart = ["apple"]
if cart:
    print("You have", len(cart), "item(s)")

Writing if cart: instead of if len(cart) > 0: is the Pythonic way to ask "is this non-empty?".

and / or return a value

This surprises many people: and and or don't always return True/False. They return one of their operands:

• a or b returns a if a is truthy, otherwise b
• a and b returns a if a is falsy, otherwise b

print("" or "default")
print("Asha" or "default")
print(0 and 99)
print(5 and 99)

This gives a neat way to provide default values:

user_input = ""
name = user_input or "Guest"
print("Welcome,", name)

Comparing with None

To check for None, use is, not ==:

result = None
print(result is None)
print(result is not None)

is checks whether two names refer to the very same object, and there is only ever one None.