What are dunder (magic) methods, and how do they enable operator overloading?

Difficulty: 5/10

Dunder Methods, Operator Overloading, Data Model

Dunder methods are hooks the data model calls for syntax and built-ins

Dunder methods (double underscore before and after) are the hooks Python's data model invokes when you use syntax or built-ins. len(x) calls type(x).len, x + y calls add, x[i] calls getitem, str(x) calls str, and with calls enter/exit. The interpreter looks these up on the type, not the instance, for most operators, which is a performance optimization and prevents accidental shadowing. Operator overloading in Python is therefore just implementing the right dunder with the correct return convention. The reflected methods (radd, rsub, etc.) handle the case where the left operand does not implement the operation.

1
Implement add, sub, mul for arithmetic; implement radd and friends for reflected operands.
2
Implement eq, lt, hash for comparisons and hashability. Use functools.total_ordering to generate the rest.
3
Implement len, getitem, iter for sequence-like behavior.
4
Trade-off: overloading makes code expressive but can surprise readers if semantics are unintuitive. Follow the principle of least surprise.
5
Common mistake: implementing eq without hash, which makes the class unhashable by setting hash to None.
6
Common mistake: assuming dunder lookup falls back to the instance. For implicit invocations it does not, except for a small set of special cases.
7
Version note: the set of dunders has grown over time, e.g., matmul in 3.5, class_getitem in 3.7, and init_subclass in 3.6.

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Scenario Questions

0-2 years experience

1You want len(myobj) to work. Which dunder do you implement?
2You want myobj[0] to work. Which dunder do you implement?

2-5 years experience

1You define __eq__ and your objects stop working as dict keys. Why?
2You want 3 + myobj to work when myobj has no __add__ for int. Which dunder handles the reversed case?

5-8 years experience

1You are designing a Money type. Which dunders do you need for arithmetic, comparison, and hashing, and what invariants do you enforce?
2Your class implements __getattr__ for lazy loading and __getitem__ for indexing. How do these interact with each other and with __getattribute__?

8+ years experience

1Design an immutable value type that supports arithmetic, comparison, hashing, pickling, and weak references, listing every dunder required and the invariants.
2Explain how dunder lookup on the type interacts with metaclasses, __getattr__, and C extension types, with examples of where the rule breaks.

Follow-up Questions

Why does Python look up dunders on the type rather than the instance?
What happens if you define __eq__ but not __hash__?

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