Modules/_abc module

Source:

cpython 3.14 @ ab2d84fe1023/Modules/_abc.c ↗

_abc is the C acceleration for abc.ABCMeta. The pure-Python fallback is in Lib/_py_abc.py. The C module provides the same interface but avoids Python-level attribute lookups in the hot isinstance path.

Map

Lines	Symbol	Role
1-100	_ABCData	Per-class state (registered subclasses, caches)
101-300	_abc_register	Add a virtual subclass
301-450	_abc_subclasscheck	ABCMeta.__subclasscheck__ implementation
451-600	_abc_instancecheck, cache management	isinstance fast path

Reading

Per-class ABC data

// CPython: Modules/_abc.c:42 _ABCData
typedef struct {
    PyObject *_abc_registry;      /* set of explicitly registered subclasses */
    PyObject *_abc_cache;         /* set of positive subclass check results */
    PyObject *_abc_negative_cache; /* set of negative results */
    Py_ssize_t _abc_negative_cache_version;
} _ABCData;

The two caches avoid repeated __subclasshook__ calls. The negative cache is versioned so it is invalidated whenever register() is called.

register

// CPython: Modules/_abc.c:155 _abc_register
static PyObject *
_abc_register(PyObject *self, PyObject *args)
{
    PyObject *subclass;
    if (!PyArg_ParseTuple(args, "O!:register", &PyType_Type, &subclass))
        return NULL;
    _ABCData *impl = _get_impl(self);
    if (PySet_Add(impl->_abc_registry, subclass) < 0)
        return NULL;
    /* Invalidate negative cache */
    abc_invalidation_counter++;
    impl->_abc_negative_cache_version = -1;
    return Py_NewRef(subclass);   /* register returns the subclass */
}

__subclasscheck__

// CPython: Modules/_abc.c:310 _abc_subclasscheck
static int
_abc_subclasscheck(PyObject *self, PyObject *subclass)
{
    /* 1. Check positive cache */
    if (PySet_Contains(impl->_abc_cache, subclass))
        return 1;
    /* 2. Check negative cache (if version matches) */
    ...
    /* 3. Call __subclasshook__ */
    PyObject *ok = PyObject_CallMethodOneArg(self, &_Py_ID(__subclasshook__), subclass);
    if (ok == Py_True) { cache_positive; return 1; }
    if (ok == Py_False) { cache_negative; return 0; }
    /* 4. Check MRO and registry */
    ...
}

The caching means that isinstance(obj, SomeABC) is O(1) after the first call for a given subclass.

gopy notes

abc.ABCMeta is used by io, collections.abc, numbers, and many other stdlib modules. In gopy ABCMeta is implemented in module/abc/ and the __subclasshook__ dispatch is handled in objects/type.go. The subclass registry is a Python set stored as a field on the ABC class object.