"Python Operator Overloading: Classes That Feel Native"
"Python operator overloading lets your classes speak the same `+`, `==`, `<` language as built-ins. This deep guide builds a Money class and a Vector class step by step, covers reflected and in-place operators, and shows where most tutorials leave landmines."
Last updated: 2026-10-05 wallet1 + wallet2 should just work. Instead, Python operator overloading throws TypeError: unsupported operand type(s) for + at you, and most tutorials stop there — one __add__ example, a mapping table, done. What they skip is exactly what breaks your code in production: the __eq__ method that silently unhashes your class, the 3 + money case that needs a reflected operator, and the wrong-type operand that should return NotImplemented instead of exploding. This guide builds two real value objects — Money and Vector — and wires up every operator you actually need. What you'll learn How Python dispatches operators to dunder methods (and the fallback chain when the first attempt fails) How to build a Money value object with working __add__ , __sub__ , and __eq__ Why defining __eq__ silently sets __hash__ to None — and how to fix it How functools.total_ordering gives you six comparisons from two methods When reflected operators like __radd__ kick in, and why you must return NotImplemented When to define __iadd__ — and when the default fallback is exactly right Python operator overloading: the data model in one mental picture Every operator in Python is a method call in disguise. a + b is a.__add__(b) . a < b is a.__lt__(b) . a[0] is a.__getitem__(0) . There is no magic — the interpreter looks up a dunder method by name and calls it. If the left operand's method can't handle the right operand, it returns the special singleton NotImplemented , and Python tries the reflected method on the right operand ( b.__radd__(a) ). Only if both sides give up do you get TypeError . That two-step dance is the key to everything below. Here are the dunders you'll actually reach for (the full data-model list is longer, but these cover ~95% of real…