Special methods are the protocol that the interpreter uses to
implement built-in operations. A type implements an operation by
defining a dunder method; the VM dispatches to that method through
a slot on the type. This page is the complete table.
Source-of-record: Objects/typeobject.c (slot setup), the
Objects/abstract.c abstract API, and
the special method names index.
Slot tables
Every type has a flat C struct of "slot" function pointers. A
Python dunder method on the class populates the matching slot; the
VM calls the slot, never reads __dict__ directly. The five major
slot tables are:
| Table | Prefix | Holds |
|---|
| Type slots | tp_ | General lifecycle, attribute, repr, etc. |
| Number methods | nb_ | +, -, *, /, integer protocols. |
| Sequence methods | sq_ | len, getitem, contains, etc. |
| Mapping methods | mp_ | len, getitem, setitem for mappings. |
| Async methods | am_ | __await__, __aiter__, __anext__. |
| Buffer methods | bf_ | __buffer__, __release_buffer__. |
Construction and destruction
| Method | Slot | When invoked |
|---|
__new__(cls, *a, **k) | tp_new | Construct a new instance. Class method. |
__init__(self, *a, **k) | tp_init | Initialise the instance. |
__del__(self) | tp_finalize | Finalisation, before deallocation. |
__init_subclass__(cls, **kw) | - | Called when a class is subclassed. |
__set_name__(self, owner, name) | - | Called on descriptors during class creation. |
Representation
| Method | Slot | Operation |
|---|
__repr__(self) | tp_repr | repr(x), fallback for str(x). |
__str__(self) | tp_str | str(x), print(x). |
__bytes__(self) | - | bytes(x). |
__format__(self, spec) | - | format(x, spec), f"{x:spec}". |
__fspath__(self) | - | os.fspath(x). |
Comparison
| Method | Slot | Operator |
|---|
__lt__ | tp_richcompare (Py_LT) | < |
__le__ | tp_richcompare (Py_LE) | <= |
__eq__ | tp_richcompare (Py_EQ) | == |
__ne__ | tp_richcompare (Py_NE) | != |
__gt__ | tp_richcompare (Py_GT) | > |
__ge__ | tp_richcompare (Py_GE) | >= |
__hash__(self) | tp_hash | hash(x). Mutable containers set this to None. |
__bool__(self) | nb_bool | bool(x). Falls back to __len__. |
Returning NotImplemented from a reflected comparison method tells
the VM to try the other side.
Attribute access
| Method | Slot | Used for |
|---|
__getattr__(self, name) | - | Fallback when normal lookup fails. |
__getattribute__(self, name) | tp_getattro | Always invoked for x.name. |
__setattr__(self, name, value) | tp_setattro | x.name = value. |
__delattr__(self, name) | tp_setattro | del x.name. |
__dir__(self) | - | dir(x). |
Descriptors
| Method | Slot | Role |
|---|
__get__(self, obj, objtype=None) | tp_descr_get | Attribute read. |
__set__(self, obj, value) | tp_descr_set | Attribute write. |
__delete__(self, obj) | tp_descr_set | Attribute delete. |
Defining __set__ or __delete__ makes the descriptor a data
descriptor and gives it priority over instance __dict__.
Container protocols
Length
| Method | Slot | Operation |
|---|
__len__(self) | sq_length / mp_length | len(x). Result must be >=0. |
__length_hint__(self) | - | operator.length_hint(x) for iterator sizing. |
Item access
| Method | Slot | Operation |
|---|
__getitem__(self, key) | mp_subscript / sq_item | x[key] |
__setitem__(self, key, value) | mp_ass_subscript / sq_ass_item | x[key] = value |
__delitem__(self, key) | mp_ass_subscript / sq_ass_item | del x[key] |
__missing__(self, key) | - | dict.__getitem__ fallback for missing keys. |
Iteration
| Method | Slot | Operation |
|---|
__iter__(self) | tp_iter | iter(x), for x in obj. |
__next__(self) | tp_iternext | next(iter). Raises StopIteration. |
__reversed__(self) | - | reversed(x). |
__contains__(self, item) | sq_contains | item in x. |
Numeric protocols
Binary arithmetic
Each binary op has a forward, reflected, and in-place form:
| Operator | Forward | Reflected | In-place |
|---|
+ | __add__ | __radd__ | __iadd__ |
- | __sub__ | __rsub__ | __isub__ |
* | __mul__ | __rmul__ | __imul__ |
/ | __truediv__ | __rtruediv__ | __itruediv__ |
// | __floordiv__ | __rfloordiv__ | __ifloordiv__ |
% | __mod__ | __rmod__ | __imod__ |
** | __pow__ | __rpow__ | __ipow__ |
@ | __matmul__ | __rmatmul__ | __imatmul__ |
<< | __lshift__ | __rlshift__ | __ilshift__ |
>> | __rshift__ | __rrshift__ | __irshift__ |
& | __and__ | __rand__ | __iand__ |
| | __or__ | __ror__ | __ior__ |
^ | __xor__ | __rxor__ | __ixor__ |
divmod | __divmod__ | __rdivmod__ | - |
__pow__ accepts a third modulus argument for pow(a, b, c).
Unary arithmetic and conversion
| Method | Slot | Operation |
|---|
__neg__(self) | nb_negative | -x |
__pos__(self) | nb_positive | +x |
__abs__(self) | nb_absolute | abs(x) |
__invert__(self) | nb_invert | ~x |
__complex__(self) | - | complex(x). |
__int__(self) | nb_int | int(x). |
__float__(self) | nb_float | float(x). |
__index__(self) | nb_index | operator.index(x). Required for slice indices, hex(), etc. |
__round__(self, ndigits=None) | - | round(x, ndigits). |
__trunc__(self) | - | math.trunc(x). |
__floor__(self) | - | math.floor(x). |
__ceil__(self) | - | math.ceil(x). |
Coroutines and async
| Method | Slot | Operation |
|---|
__await__(self) | am_await | await x. Returns an iterator. |
__aiter__(self) | am_aiter | async for x in obj. Returns async iterator. |
__anext__(self) | am_anext | Next async-iteration value. |
__aenter__(self) | - | async with enter. |
__aexit__(self, et, ev, tb) | - | async with exit. |
Context managers
| Method | Operation |
|---|
__enter__(self) | with x as y: enter. |
__exit__(self, exc_type, exc_val, exc_tb) | with exit. |
Generator and coroutine flow
| Method | Operation |
|---|
send(self, value) | Resume a generator and send a value. |
throw(self, type[, value[, tb]]) | Raise an exception inside the generator. |
close(self) | Raise GeneratorExit. |
Pattern matching (PEP 634)
| Attribute / method | Role |
|---|
__match_args__ | Tuple of attribute names for positional class patterns. |
__instancecheck__(cls, instance) | isinstance(instance, cls). |
__subclasscheck__(cls, subclass) | issubclass(subclass, cls). |
Reflection support
| Method | Operation |
|---|
__sizeof__(self) | sys.getsizeof(x). |
__dir__(self) | dir(x). |
__class_getitem__(cls, item) | cls[item]. Class-level subscript (e.g. list[int]). |
__instancecheck__(cls, obj) | Custom isinstance. |
__subclasscheck__(cls, sub) | Custom issubclass. |
__init_subclass__(cls, **kw) | Customise subclass creation. |
Class creation
| Method | Operation |
|---|
__prepare__(cls, name, bases, **kw) | Returns the namespace mapping. Metaclass method. |
__init_subclass__(cls, **kw) | Run when a subclass is defined. |
__set_name__(self, owner, name) | Called on descriptors at class creation. |
__class_getitem__(cls, item) | Supports list[int]. |
__mro_entries__(self, bases) | Hook to replace a base in __bases__. |
Copying
The C-level slots do not implement copying; the protocol lives in
copy.py:
| Method | Used by |
|---|
__copy__(self) | copy.copy. |
__deepcopy__(self, memo) | copy.deepcopy. |
__reduce__(self) | Pickling fallback. |
__reduce_ex__(self, p) | Versioned pickling hook. |
__getstate__(self) | Pickling state extraction. |
__setstate__(self, s) | Pickling state restoration. |
__getnewargs__(self) | Pickling __new__ args. |
__getnewargs_ex__(self) | Pickling __new__ args with kwargs. |
Buffer protocol
| Method | Slot | Role |
|---|
__buffer__(self, flags) | bf_getbuffer | Return a memoryview. |
__release_buffer__(self, mv) | bf_releasebuffer | Release acquired buffer. |
Call
| Method | Slot | Operation |
|---|
__call__(self, *args, **kwargs) | tp_call | x(...). |
Type-parameter protocols (PEP 695)
| Attribute | Meaning |
|---|
__type_params__ | Tuple of TypeVar, TypeVarTuple, ParamSpec. |
__typing_subst__ | Used by Generic during substitution. |
Gopy status
Every dunder in CPython 3.14 dispatches to the same slot in gopy.
The list above is exhaustive; if a method is reachable from CPython
it is reachable from gopy unless flagged in Status.
Reference
Objects/typeobject.c. The slot-method wiring.objects/typeobject.go. gopy's port.- Data model for the lookup rules.
- Built-in types for which slots each type implements.