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子类方法是如何在CPython中实现的?

python-internals python-3.x python

max · 技术社区 · 8 年前

文件说:

每个类都保留对其直接子类的弱引用列表。此方法返回所有仍然有效的引用的列表。

但是,每个类如何首先获得对其子类的弱引用列表?换句话说,当我创建

class B(A):
  pass

怎么做 A 发现 B 只是子类化了它?这种机制是否足够健壮,能够在边缘情况下生存(自定义元类、分配到 __bases__ 等等)?

1 回复 | 直到 8 年前

user2357112 8 年前

作为新类初始化的一部分,将该类的弱引用添加到 tp_subclasses 每个基类的成员。您可以在中的Python源代码中看到这一点 Objects/typeobject.c :

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;
}

这个 setter 对于 __bases__ 还更新每个旧基和新基的子类列表:

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);
    }
    ...
}

注意,如果元类做了一些事情来定制子类关系的含义, __subclasses__ 不会反映这一点。例如 issubclass(list, collections.abc.Iterable) 是 True 但是 list 不会在搜索中显示 __子类__ collections.abc.Iterable .