Lib/_collections_abc.py

Source:

cpython 3.14 @ ab2d84fe1023/Lib/_collections_abc.py ↗

Lib/_collections_abc.py defines the abstract base classes (ABCs) for Python's container types. These ABCs are re-exported as collections.abc.*. Each ABC uses __subclasshook__ for virtual subclass registration (so a class implementing the required methods is considered a subclass even without explicit inheritance) and provides mixin methods built from the abstract ones.

Map

Lines	Symbol	Role
1-80	ABCMeta, helper	Metaclass import; _check_methods
81-200	Hashable, Iterable, Iterator, Reversible	Primitive ABCs
201-400	Sized, Container, Collection	Size/containment ABCs
401-600	Sequence, MutableSequence	Ordered sequence ABCs with mixin methods
601-750	Mapping, MutableMapping	Dict-like ABCs
751-900	Set, MutableSet	Set ABCs with set-arithmetic mixins
901-1000	ByteString, Buffer, registration	Bytes-like ABCs; virtual registrations

Reading

subclasshook for duck typing

Iterable.__subclasshook__ returns True if the class has an __iter__ method, making any class with __iter__ a virtual subclass of Iterable without explicit inheritance. _check_methods(C, *methods) is the helper that walks the MRO checking for the presence of each method name.

# CPython: Lib/_collections_abc.py:81 Iterable.__subclasshook__
class Iterable(metaclass=ABCMeta):
    @abstractmethod
    def __iter__(self): ...

    @classmethod
    def __subclasshook__(cls, C):
        if cls is Iterable:
            return _check_methods(C, '__iter__')
        return NotImplemented

MutableSequence mixin methods

MutableSequence declares __getitem__, __setitem__, __delitem__, __len__, and insert as abstract, then provides concrete implementations of append, clear, reverse, extend, pop, __iadd__, and __contains__ in terms of the abstract methods.

# CPython: Lib/_collections_abc.py:401 MutableSequence.append
class MutableSequence(Sequence):
    def append(self, value):
        self.insert(len(self), value)

    def pop(self, index=-1):
        v = self[index]
        del self[index]
        return v

Set arithmetic mixins

Set provides __and__, __or__, __xor__, __sub__, and their reflected variants as mixin methods built from __contains__, __iter__, and __len__. MutableSet adds add, discard, and __ior__, __iand__, __ixor__, __isub__.

gopy notes

Not yet ported. In Go, interface types serve the same role as ABCs; there is no virtual subclass registration mechanism. A faithful port would use Go interfaces for the protocol checks and ignore __subclasshook__ since Go's type system does not need it.