Python __new__ - cls 如何与 __new__ 所在的类不同

Question

Documentation for object.__new__(cls[, ...]) says:

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.

因此，如果您有 var = MyClass() 和

class MyClass:
    def __new__(cls):
        ...

cls 不会总是等于 MyClass 吗？

Answer 1

试试这个:

class MyClass:
    def __new__(cls):
        print(cls)

class Yac(MyClass):
    pass

x = Yac()
print(x)

我们得到:

<class '__main__.Yac'>
None

顺便说一下，这在 Python 3 中没问题，但在 Python 2 中不起作用。在 Python 2 中，__new__ 仅在“新式”类中受支持:

class MyClass(object):

在 Python 3 中，所有类都是“新式”的。