python - 为什么 __new__ 没有类方法装饰器

Question

为什么 new 方法没有类装饰器？例如:

class MyClass:
    @classmethod
    def __new__(cls, *args, **kwargs):
        pass

这个方法如何能够在没有装饰器的情况下充当 classmethod？

Answer 1

它是特别特殊的。 type.__new__ 将在创建类时自动应用 staticmethod 如果它发现您定义了一个 __new__ 而没有应用 staticmethod . (此外，__new__ 应该是 staticmethod，而不是 classmethod。)

参见 docs :

Called to create a new instance of class cls. __new__() is a static method (special-cased so you need not declare it as such) that takes the class of which an instance was requested as its first argument...

和 implementation :

    /* Special-case __new__: if it's a plain function,
       make it a static function */
    tmp = _PyDict_GetItemId(dict, &PyId___new__);
    if (tmp != NULL && PyFunction_Check(tmp)) {
        tmp = PyStaticMethod_New(tmp);
        if (tmp == NULL)
            goto error;
        if (_PyDict_SetItemId(dict, &PyId___new__, tmp) < 0) {
            Py_DECREF(tmp);
            goto error;
        }
        Py_DECREF(tmp);
    }