Lib/abc.py (part 5)

Source:

cpython 3.14 @ ab2d84fe1023/Lib/abc.py ↗

This annotation covers the abstract base class mechanism. See objects_typeobject12_detail for isinstance/issubclass which integrates with ABCs.

Map

Lines	Symbol	Role
1-60	ABCMeta.__new__	Create a new abstract class
61-120	ABCMeta.register	Register a virtual subclass
121-200	ABCMeta.__subclasscheck__	Override issubclass for virtual subclassing
201-300	abstractmethod	Decorator marking a method as abstract

Reading

ABCMeta.__new__

# CPython: Lib/abc.py:85 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

_abc_init is implemented in C (_abc module) and initializes the _abc_data structure. It collects the set of abstract method names from the class and all bases.

ABCMeta.register

# CPython: Lib/abc.py:100 ABCMeta.register
def register(cls, subclass):
    if not isinstance(subclass, type):
        raise TypeError("Can only register classes")
    if issubclass(subclass, cls):
        return subclass   # already a subclass
    cls._abc_registry.add(subclass)
    ABCMeta._abc_invalidation_counter += 1
    return subclass

Iterable.register(MyClass) makes isinstance(MyClass(), Iterable) return True even though MyClass doesn't inherit from Iterable. This is "virtual subclassing".

ABCMeta.__subclasscheck__

# CPython: Lib/_py_abc.py:90 __subclasscheck__
def __subclasscheck__(cls, subclass):
    # Check subclass hook first
    ok = cls.__subclasshook__(subclass)
    if ok is not NotImplemented:
        return ok
    # Check registered virtual subclasses
    if subclass in cls._abc_cache:
        return True
    # Walk the MRO of subclass
    for scls in subclass.__mro__:
        if scls in cls._abc_registry:
            cls._abc_cache.add(subclass)
            return True
    return False

__subclasshook__ is tried first, allowing ABCs like Iterable to accept any class with an __iter__ method. The result is cached in _abc_cache for performance.

abstractmethod

# CPython: Lib/abc.py:7 abstractmethod
def abstractmethod(funcobj):
    funcobj.__isabstractmethod__ = True
    return funcobj

@abstractmethod sets __isabstractmethod__ = True on the function. ABCMeta.__new__ collects all names with this attribute into __abstractmethods__. Instantiating a class with non-empty __abstractmethods__ raises TypeError.

gopy notes

ABCMeta is objects.ABCMeta in objects/type.go. register adds to objects.ABCData.Registry. __subclasscheck__ checks the cache and registry. abstractmethod sets the IsAbstract flag on objects.Function.