Lib/abc.py

cpython 3.14 @ ab2d84fe1023/Lib/abc.py ↗

Lib/abc.py is the public surface of Python's abstract-base-class machinery. The real implementation lives in _abc (a C extension), while this file provides the Python-level decorators, metaclass, and the convenience ABC base class that most user code sees.

Map

Lines	Symbol	Role
1-25	imports, ABCMeta.__new__	Delegates to _abc_init
26-80	ABCMeta	Metaclass with register, __subclasscheck__, __instancecheck__
81-110	ABC	Convenience base that sets metaclass=ABCMeta
111-160	abstractmethod	Decorator that sets __isabstractmethod__ = True
161-184	abstractclassmethod, abstractstaticmethod, abstractproperty	Deprecated combined decorators

Reading

ABCMeta and the _abc C backend

ABCMeta.__new__ calls _abc_init(cls) immediately after the class is created by the normal type.__new__ machinery. _abc_init sets up the per-class registry and cache that __subclasscheck__ consults.

# CPython: Lib/abc.py:27 ABCMeta.__new__
class ABCMeta(type):
    def __new__(mcls, name, bases, namespace, /, **kwargs):
        cls = super().__new__(mcls, name, bases, namespace, **kwargs)
        _abc_init(cls)
        return cls

register and virtual subclasses

ABCMeta.register records a class as a virtual subclass without modifying the class hierarchy. The bookkeeping lives entirely in the _abc C module; this wrapper just calls through and clears the negative cache.

# CPython: Lib/abc.py:40 ABCMeta.register
def register(cls, subclass):
    return _abc_register(cls, subclass)

abstractmethod

Setting __isabstractmethod__ = True on a function is the only mechanism. type.__new__ scans the namespace for functions with that attribute set to determine whether a class is abstract.

# CPython: Lib/abc.py:116 abstractmethod
def abstractmethod(funcobj):
    funcobj.__isabstractmethod__ = True
    return funcobj

__subclasshook__

If a class defines __subclasshook__, ABCMeta.__subclasscheck__ calls it first. If it returns NotImplemented, the check falls through to the ordinary MRO walk and virtual subclass registry.

gopy notes

Not yet ported. The ABCMeta metaclass requires the type system to support custom __instancecheck__ and __subclasscheck__ dispatch, which gopy's type layer handles through objects/type.go. A thin module/abc/ wrapper can expose ABC and abstractmethod once that dispatch is stable.