All 15 Magic Methods & Operator Overloading questions
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What are 'magic methods' (dunder methods) in Python, and why are they called that? What is the difference between __str__ and __repr__? How does __repr__ help with debugging when printing a list of custom objects? How do you overload arithmetic operators like + and - using __add__ and __sub__? 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 does __contains__ let you customize the behavior of the 'in' operator? 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?
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What do __eq__ and __hash__ need to satisfy together, and what happens if you only override __eq__?

Intermediate

Answer

If two objects are equal (__eq__ returns True), they must have the same hash value so they behave consistently in sets and dict keys. By default, defining __eq__ without __hash__ makes the class unhashable (Python sets __hash__ to None automatically), which raises TypeError if you try to put instances in a set or use them as dict keys.
class Point:
    def __init__(self, x, y):
        self.x, self.y = x, y
    def __eq__(self, other):
        return (self.x, self.y) == (other.x, other.y)
    def __hash__(self):
        return hash((self.x, self.y))

{Point(1,2)}  # works because __hash__ is defined
Real-world example A caching layer uses custom key objects as dict keys, so their class must define both __eq__ and __hash__ consistently or lookups will fail unpredictably.

Common follow-ups: Why does Python disable __hash__ when __eq__ is overridden?;How does this interact with mutable objects?

Data Types & Structures;Dataclasses & NamedTuples
All 15 Magic Methods & Operator Overloading questions