What are dunder (magic) methods in Python?

Question

Accepted Answer

Dunder (“double underscore”) methods — also called magic or special methods — are the methods Python calls implicitly to implement language-level behavior on your objects: operators, built-ins, iteration, context management, attribute access, and more. Their names begin and end with __ (e.g. __init__ , __len__ , __add__ ). The data model: defining these methods opts your class into Python’s data model . Instead of calling obj.__len__() , users call len(obj) ; instead of a.__add__(b) , they write a + b . Your class behaves like a built-in. Common categories: Construction & lifecycle — __new__ , __init__ , __del__ . String representations — __repr__ (developer-facing, aim to make it unambiguous), __str__ (user-facing), __format__ . Equality & ordering — __eq__ , __hash__ , __lt__ , __le__ , __gt__ , __ge__ ; consider @functools.total_ordering . Numeric operators — __add__ , __sub__ , __mul__ , __truediv__ , plus reflected ( __radd__ ) and in-place ( __iadd__ ) variants. Containers & iteration — __len__ , __getitem__ , __setitem__ , __delitem__ , __iter__ , __next__ , __contains__ , __reversed__ . Callable — __call__ makes an instance callable like a function. Context managers — __enter__ , __exit__ for with blocks. Attribute access — __getattr__ , __getattribute__ , __setattr__ , __delattr__ ; descriptors via __get__ , __set__ , __delete__ . Async — __await__ , __aiter__ , __anext__ , __aenter__ , __aexit__ . Introspection & meta — __class__ , __dict__ , __slots__ , __init_subclass__ , __set_name__ Know the most common dunder methods: __init__, __str__, __repr__, __eq__, __hash__, __len__, __add__, __iter__. The __str__ vs __repr__ distinction (human-readable vs developer-readable) is frequently asked. Show operator overloading (__add__ for +) and context manager (__enter__/__exit__ for with).