Python Booleans

Python provides the bool type, which can have two values: True and False (capitalized)

done = False
done = True

Booleans are especially useful with conditional control structures like if statements:

done = True

if done:
    # run some code here
else:
    # run some other code

Notice we write if done:, not if done == True:. The comparison is redundant, since done already holds a boolean.

Truthy and falsy values

When evaluating a value for True or False, if the value is not a bool we have some rules depending on the type we’re checking:

• numbers are always True except for 0 (and 0.0)
• strings are False only when empty
• lists, tuples, sets, dictionaries are False only when empty

You can check how any value evaluates by passing it to the bool() function:

bool(0)        # False
bool(3.14)     # True
bool('')       # False
bool('hello')  # True
bool([])       # False

This matters in conditionals. Writing if items: runs the code only when the items list has at least one element.

Checking if a value is a boolean

You can check if a value is a boolean in this way:

done = True
type(done) == bool # True

Or using isinstance(), passing 2 arguments: the variable, and the bool class:

done = True
isinstance(done, bool) # True

Booleans are numbers, too

In Python, bool is a subclass of int. True behaves as 1 and False as 0:

True + True # 2
True * 8    # 8

This has a practical use: sum() counts how many values in a list are True:

answers = [True, False, True, True]
sum(answers) # 3

But it also hides a pitfall: isinstance(True, int) returns True. If you’re inspecting types and need to tell integers and booleans apart, check for bool first, before checking for int.

The any() and all() functions

The global any() function is also very useful when working with booleans, as it returns True if any of the values of the iterable (a list, for example) passed as argument are True:

book_1_read = True
book_2_read = False

read_any_book = any([book_1_read, book_2_read])

The global all() function is similar, but returns True only if all of the values passed to it are True:

ingredients_purchased = True
meal_cooked = False

ready_to_serve = all([ingredients_purchased, meal_cooked])

Here ready_to_serve is False, because meal_cooked is False.