All 15 Magic Methods & Operator Overloading questions
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What do __eq__ and __hash__ need to satisfy together, and what happens if you only override __eq__? How do __len__, __getitem__, and __setitem__ let a custom class act like a sequence or mapping? How does __call__ let an instance of a class be invoked like a function? How do comparison magic methods like __lt__, __le__, __gt__, and __ge__ support custom sorting? How do you implement custom attribute access with __getattr__ versus __getattribute__? How do context manager magic methods __enter__ and __exit__ work, and what does __exit__'s return value control? What is the difference between __new__ and __init__, and when would you override __new__? How do __iadd__ and other in-place operator methods (__isub__, __imul__) differ from their non-in-place counterparts? How does __bool__ (or the legacy __nonzero__) determine an object's truthiness, and what's the fallback if it's not defined? How does __slots__ interact with magic methods and instance attribute storage?
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Python › Magic Methods & Operator Overloading

How does __contains__ let you customize the behavior of the 'in' operator?

Intermediate

Answer

Defining __contains__(self, item) lets Python call it directly when evaluating `item in obj`, instead of falling back to iterating the whole object with __iter__ or __getitem__. This can be much faster for containers with efficient membership tests, like a class backed by a set or a database query.
class UserGroup:
    def __init__(self, user_ids):
        self._ids = set(user_ids)  # O(1) lookup
    def __contains__(self, user_id):
        return user_id in self._ids

group = UserGroup([1,2,3])
print(2 in group)  # True, O(1) via set
Real-world example A permissions class backed by a set of allowed role IDs implements __contains__ so `if role in permissions:` checks run in constant time instead of scanning a list.

Common follow-ups: What's the fallback behavior if __contains__ isn't defined?;How does this interact with __iter__ for custom iterables?

Data Types & Structures;Iterators & the Iterator Protocol
All 15 Magic Methods & Operator Overloading questions