Bools in

About Bools

Python represents true and false values with the bool type, which is a subtype of int. There are only two Boolean values in this type: True and False. These values can be assigned to a variable and combined with the Boolean operators (and, or, not):

>>> true_variable = True and True
>>> false_variable = True and False

>>> true_variable = False or True
>>> false_variable = False or False

>>> true_variable = not False
>>> false_variable = not True

Boolean operators use short-circuit evaluation, which means that expression on the right-hand side of the operator is only evaluated if needed.

Each of the operators has a different precedence, where not is evaluated before and and or. Brackets can be used to evaluate one part of the expression before the others:

>>>not True and True
False

>>>not (True and False)
True

All boolean operators are considered lower precedence than Python's comparison operators, such as ==, >, <, is and is not.

Type Coercion and Truthiness

The bool function (bool()) converts any object to a Boolean value. By default all objects return True unless defined to return False.

A few built-ins are always considered False by definition:

• the constants None and False
• zero of any numeric type (int, float, complex, decimal, or fraction)
• empty sequences and collections (str, list, set, tuple, dict, range(0))

>>>bool(None)
False

>>>bool(1)
True

>>>bool(0)
False

>>>bool([1,2,3])
True

>>>bool([])
False

>>>bool({"Pig" : 1, "Cow": 3})
True

>>>bool({})
False

When an object is used in a boolean context, it is evaluated transparently as truthy or falsey using bool():

>>> a = "is this true?"
>>> b = []

# This will print "True", as a non-empty string is considered a "truthy" value
>>> if a:
...  print("True")

# This will print "False", as an empty list is considered a "falsey" value
>>> if not b:
...   print("False")

Classes may define how they are evaluated in truthy situations if they override and implement a __bool__() method, and/or a __len__() method.

How Booleans work under the hood

The bool type is implemented as a sub-type of int. That means that True is numerically equal to 1 and False is numerically equal to 0. This is observable when comparing them using an equality operator:

>>>1 == True
True

>>>0 == False
True

However, bools are still different from ints, as noted when comparing them using the identity operator, is:

>>>1 is True
False

>>>0 is False
False

Note: in python >= 3.8, using a literal (such as 1, '', [], or {}) on the left side of is will raise a warning.

It is considered a Python anti-pattern to use the equality operator to compare a boolean variable to True or False. Instead, the identity operator is should be used:

>>> flag = True

# Not "Pythonic"
>>> if flag == True:
...    print("This works, but it's not considered Pythonic.")

# A better way
>>> if flag:
...    print("Pythonistas prefer this pattern as more Pythonic.")