__subclasses__ 方法在 CPython 中是如何实现的？

Question

文档说：

每个类都有一个对其直接子类的弱引用列表。此方法返回所有仍然存在的引用的列表。

但是每个类首先如何获得对其子类的弱引用列表呢？换句话说，当我创建

class B(A):
  pass

A 如何发现 B 只是对其进行子类化？这种机制是否足够强大，可以在边缘情况下生存（自定义元类、分配给__bases__ 等）？

Answer 1

作为新类初始化的一部分，对该类的弱引用被添加到其每个基类的tp_subclasses 成员中。你可以在Objects/typeobject.c的Python源代码中看到这一点：

int
PyType_Ready(PyTypeObject *type)
{
    ...
    /* Link into each base class's list of subclasses */
    bases = type->tp_bases;
    n = PyTuple_GET_SIZE(bases);
    for (i = 0; i < n; i++) {
        PyObject *b = PyTuple_GET_ITEM(bases, i);
        if (PyType_Check(b) &&
            add_subclass((PyTypeObject *)b, type) < 0)
            goto error;
    }
    ...
}

static int
add_subclass(PyTypeObject *base, PyTypeObject *type)
{
    int result = -1;
    PyObject *dict, *key, *newobj;

    dict = base->tp_subclasses;
    if (dict == NULL) {
        base->tp_subclasses = dict = PyDict_New();
        if (dict == NULL)
            return -1;
    }
    assert(PyDict_CheckExact(dict));
    key = PyLong_FromVoidPtr((void *) type);
    if (key == NULL)
        return -1;
    newobj = PyWeakref_NewRef((PyObject *)type, NULL);
    if (newobj != NULL) {
        result = PyDict_SetItem(dict, key, newobj);
        Py_DECREF(newobj);
    }
    Py_DECREF(key);
    return result;
}

__bases__ 的 setter 还更新每个新旧基的子类列表：

static int
type_set_bases(PyTypeObject *type, PyObject *new_bases, void *context)
{
    ...
    if (type->tp_bases == new_bases) {
        /* any base that was in __bases__ but now isn't, we
           need to remove |type| from its tp_subclasses.
           conversely, any class now in __bases__ that wasn't
           needs to have |type| added to its subclasses. */

        /* for now, sod that: just remove from all old_bases,
           add to all new_bases */
        remove_all_subclasses(type, old_bases);
        res = add_all_subclasses(type, new_bases);
        update_all_slots(type);
    }
    ...
}

请注意，如果元类做了一些事情来自定义子类关系的含义，__subclasses__ 不会反映这一点。例如，issubclass(list, collections.abc.Iterable) 是True，但list 不会出现在从collections.abc.Iterable 开始的__subclasses__ 树的搜索中。