__bool__ — Easy Examples

Called by bool(); defines truthiness of the object

EasyIntermediateAdvancedExpert

Implementing __bool__

Basic implementation of __bool__ in a class.

python
class NonEmpty:
    def __init__(self, items):
        self.items = items

    def __bool__(self):
        return len(self.items) > 0

a = NonEmpty([1, 2, 3])
b = NonEmpty([])
print(bool(a))
print(bool(b))
if a:
    print("a is truthy")

__bool__ called by bool(); defines truthiness of the object. Implementing it lets you customize how Python interacts with your objects.

__bool__ in action

Seeing __bool__ called by Python's built-in operations.

python
# How Python calls __bool__ automatically
class Demo:
    def __init__(self, value):
        self.value = value

    def __bool__(self):
        print(f"__bool__ was called!")
        return True

d = Demo(42)
# This triggers __bool__:
print(bool(d))

Python automatically calls __bool__ when you use the corresponding operator or function on your object.

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