__contains__ — Easy Examples

Called by the 'in' operator to test membership

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Implementing __contains__

Basic implementation of __contains__ in a class.

python
class NumberRange:
    def __init__(self, start, end):
        self.start = start
        self.end = end

    def __contains__(self, item):
        return self.start <= item <= self.end

r = NumberRange(1, 10)
print(5 in r)
print(15 in r)

__contains__ called by the 'in' operator to test membership. Implementing it lets you customize how Python interacts with your objects.

__contains__ in action

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

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

    def __contains__(self, other):
        print(f"__contains__ was called!")
        return self

d = Demo(42)
# This triggers __contains__:
print(1 in d)

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

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