Reference

Dunder Methods

Python Dunder Methods reference.

1. __abs__ / __neg__ / __pos__

   Implement __abs__, __neg__, and __pos__ to control how Python's unary operators behave with your objects.

   __abs__(self) | __neg__(self) | __pos__(self)
2. __add__ / __radd__ / __iadd__

   Implement the + and += operators on your custom classes with __add__, __radd__, and __iadd__.

   def __add__(self, other)
3. __aenter__ / __aexit__

   Async context manager methods called by async with for resource setup and teardown. Define __aenter__ for entry and __aexit__ for exit logic.

4. __aiter__ / __anext__

   The async iteration protocol pair. __aiter__ returns an async iterator; __anext__ returns an awaitable each step until StopAsyncIteration is raised.

5. __await__

   The __await__ dunder method makes objects awaitable with await. Returns an iterator that Python drives to suspension and completion.

6. __bool__

   Control how your objects evaluate in boolean context — Python's __bool__ method explained with practical examples.

7. __call__

   Make instances callable like functions

   def __call__(self, ...)
8. __class_getitem__

   A class method that handles subscription on the class itself, enabling generic type expressions like MyClass[T].

   __class_getitem__(cls, item)
9. __contains__

   Implement the __contains__ method to define custom membership test behavior for the in operator in Python.

   __contains__(item)
10. __copy__ / __deepcopy__

    Implement __copy__ and __deepcopy__ to control how copy.copy() and copy.deepcopy() handle your class instances.

11. __del__

    Understand Python's __del__ destructor, its relationship to garbage collection, and why to avoid it in modern Python 3.

12. __delitem__

    Learn how to implement __delitem__ to enable custom deletion behavior with the del keyword in your Python classes.

    def __delitem__(self, key)
13. __enter__ / __exit__

    A complete reference for Python's context manager protocol, covering __enter__ and __exit__ dunder methods, exception handling, and practical examples.

14. __eq__ / __ne__

    Learn how to define equality and inequality for your Python objects using __eq__ and __ne__ dunder methods, with code examples.

    __eq__(self, other) __ne__(self, other)
15. __format__

    Define custom string formatting for your objects. Controls how they appear in f-strings and format() calls.

    __format__(self, format_spec)
16. __get__ / __set__ / __delete__

    Control how attributes are accessed, assigned, or deleted on class instances with Python's descriptor protocol: __get__, __set__, and __delete__.

    __get__(self, obj, objtype=None) -> Any
17. __getattr__ / __getattribute__

    Control Python attribute access with __getattribute__ (every lookup) and __getattr__ (missing attributes only).

18. __getitem__

    Implement subscription access for custom objects with __getitem__ — the hook behind obj[key] syntax in Python.

19. __hash__

    Define the hash value for your Python objects so they can be used in sets and as dictionary keys.

    __hash__(self)
20. __init__

    Initialize instance attributes after object creation. Called automatically after __new__ completes.

    __init__(self, /, *args, **kwargs)
21. __init_subclass__

    A hook Python calls when a class inherits from one that defines it. Runs at class definition time, not instantiation.

    __init_subclass__(cls, /, **kwargs)
22. __iter__

    Learn how to implement __iter__ so custom Python objects work with for loops, iter(), list(), and other iteration tools.

    __iter__(self)
23. __len__

    Implement __len__ to define custom length behavior for your objects with Python's __len__ dunder method.

    __len__(self)
24. __lt__ / __le__ / __gt__ / __ge__

    Learn how to define comparison operators for custom Python classes using __lt__, __le__, __gt__, and __ge__ rich comparison dunder methods.

    __lt__(self, other)
25. __matmul__ / __rmatmul__

    Implement the @ matrix multiplication operator with __matmul__, __rmatmul__, and __imatmul__ for custom objects in Python.

26. __missing__

    Called when d[key] lookup fails on a dict subclass. Return a default value or raise KeyError.

    __missing__(self, key)
27. __mul__ / __rmul__ / __imul__

    Python's three multiplication dunder methods — when each is called, how NotImplemented works, and how __imul__ differs for mutables.

28. __new__

    Creates and returns a new class instance before __init__ is called. Used to implement singletons, immutable objects, and factory patterns.

    def __new__(cls, *args, **kwargs)
29. __next__

    Learn how to implement the __next__ method to create custom iterators in Python. Complete guide with examples.

    __next__(self)
30. __reduce__ / __reduce_ex__

    Control how Python serializes and reconstructs your objects during pickling, copying, and shelving.

31. __repr__

    Return a string representation of an object for debugging. Defined automatically for all objects.

    __repr__(self)
32. __set_name__

    Called automatically when a class containing the descriptor is defined. Sets the owner class and attribute name on the descriptor instance.

    __set_name__(self, owner, name)
33. __setattr__ and __delattr__

    Control attribute assignment and deletion with Python's __setattr__ and __delattr__ dunder methods.

34. __setitem__

    Python's __setitem__ method enables subscript assignment. Learn how to implement key-value setting in custom classes.

    __setitem__(self, key, value)
35. __sizeof__

    Returns the size of an object in bytes. Measures the object's own memory footprint, not what it references.

    __sizeof__(self)
36. __slots__

    Declare fixed instance attributes in Python classes to eliminate __dict__ overhead and reduce memory usage.

    __slots__ = ['name', ...]
37. __str__

    Python's __str__ magic method explained with practical examples

    __str__(self)
38. __sub__ / __rsub__ / __isub__

    Implement subtraction in Python classes: __sub__ for a - b, __rsub__ for reversed b - a, and __isub__ for in-place a -= b.

    __sub__(self, other)
39. __truediv__ / __floordiv__

    Implements true division (/) and floor division (//) operators. Covers return types, reversed variants, and common patterns.

    __truediv__(self, other)