Lib/abc.py (supplemental)

Source:

cpython 3.14 @ ab2d84fe1023/Lib/abc.py ↗

This annotation supplements lib_abc_detail with deeper coverage of ABCMeta.__new__, the _abc C extension caching layer, and the abstract decorator family.

Map

Lines	Symbol	Role
1-20	_abc C extension import	_abc.ABCMeta, _abc._check_methods
21-80	ABCMeta	Metatype for ABCs
81-130	ABC	Convenience base class using ABCMeta as metatype
131-180	abstractmethod, abstractclassmethod, abstractstaticmethod, abstractproperty	Decorators marking abstract methods

Reading

ABCMeta.__new__ and __abstractmethods__

When ABCMeta creates a class, it scans the class namespace for any callables with __isabstractmethod__ = True (set by @abstractmethod) and stores their names in cls.__abstractmethods__ (a frozenset). A class with non-empty __abstractmethods__ cannot be instantiated.

# CPython: Lib/abc.py:65 ABCMeta.__new__
class ABCMeta(type):
    def __new__(mcls, name, bases, namespace, /, **kwargs):
        cls = super().__new__(mcls, name, bases, namespace, **kwargs)
        _abc_init(cls)  # calls _abc C extension
        return cls

_abc C extension

Modules/_abc.c provides:

• _abc_init(cls): sets up _abc_data (the negative/positive cache) on the class
• ABCMeta.register(cls, subclass): registers subclass as a virtual subclass
• ABCMeta.__subclasshook__ dispatch: checks the subclass hook before scanning the MRO
• Cache: positive and negative isinstance results are cached per ABC to avoid repeated MRO scans

abstractmethod

# CPython: Lib/abc.py:129 abstractmethod
def abstractmethod(funcobj):
    funcobj.__isabstractmethod__ = True
    return funcobj

Simply sets the __isabstractmethod__ attribute. ABCMeta.__new__ collects all such attributes from the class body and inherited bases.

__subclasshook__

A class method that can short-circuit isinstance/issubclass checks. Return True to force a positive result, False to force negative, NotImplemented to fall back to the standard check.

# CPython: Lib/abc.py:93 ABCMeta.__subclasscheck__
def __subclasscheck__(cls, subclass):
    return _abc_subclasscheck(cls, subclass)

_abc_subclasscheck (C extension) first tries cls.__subclasshook__(subclass), then checks the registered virtual subclasses, then does a standard MRO scan.

gopy notes

ABCMeta and abstractmethod are ported in objects/type.go. The _abc caching layer is approximated by storing a map[*Type]bool in the Type struct for the positive/negative cache. __abstractmethods__ is a frozenset stored in the type's __dict__.