Inheritance when __new__() doesn't return instance of class

问题

When __new__ return instance of class, everything is ok, we can create subclass with no problems:

class A:
    def __new__(cls, p1, p2):
        self = object.__new__(cls) 
        return self

    def __init__(self, p1, p2):
        self.p1 = p1
        self.p2 = p2

class B(A):
    def __new__(cls, p3):
        self = super().__new__(cls, 1, 2)
        return self

    def __init__(self, p3):
        super().__init__(1, 2)
        self.p3 = p3

a = A(1, 2)    
print(a.p2)  # output: 2

b = B(3)
print(b.p3)  # output: 3

But,

If __new__() does not return an instance of cls, then the new instance’s __init__() method will not be invoked.

Looks like we have to call __init__() inside __new__() directly, but this leads to error, when we call super().__new__ in subclass:

class A:
    def __new__(cls, p1, p2):
        self = object.__new__(cls)
        self.__init__(p1, p2)  # we should call __init__ directly
        return [self]  # return not instance

    def __init__(self, p1, p2):
        self.p1 = p1
        self.p2 = p2

class B(A):
    def __new__(cls, p3):
        self = super().__new__(cls, 1, 2)
        return self

    def __init__(self, p3):
        self.p3 = p3

a = A(1, 2)    
print(a[0].p2)  # output: 2

b = B(3)  # TypeError: __init__() takes 2 positional arguments but 3 were given
print(b[0].p3)

How to resolve it? How to create A's subclass if A.__new__() doesn't return an instance of class?

回答1:

If you are going to call it manually, either don't name the method __init__ and use per-class names instead, or manually call __init__ methods unbound, directly on the class.

Per-class names are relatively easy, you can use double-underscores at the start to produce class-specific names through name mangling:

class A:
    def __new__(cls, p1, p2):
        self = object.__new__(cls)
        self.__init(p1, p2)  # call custom __init directly
        return [self]  # return not instance

    def __init(self, p1, p2):
        self.p1 = p1
        self.p2 = p2

class B(A):
    def __new__(cls, p3):
        self = super().__new__(cls, 1, 2)
        self[0].__init(p3)  # custom init
        return self

    def __init(self, p3):
        self.p3 = p3

or directly on the class:

class A:
    def __new__(cls, p1, p2):
        self = object.__new__(cls)
        A.__init__(self, p1, p2)  # call custom __init__ unbound
        return [self]  # return not instance

    def __init__(self, p1, p2):
        self.p1 = p1
        self.p2 = p2

class B(A):
    def __new__(cls, p3):
        self = super().__new__(cls, 1, 2)
        B.__init__(self[0], p3)  # call custom __init__ unbound
        return self

    def __init__(self, p3):
        self.p3 = p3

If you are going this way, you may as well do away with a custom initialiser and just do your initialisation in __new__ altogether:

class A:
    def __new__(cls, p1, p2):
        self = object.__new__(cls)
        self.p1 = p1
        self.p2 = p2
        return [self]  # return not instance


class B(A):
    def __new__(cls, p3):
        self = super().__new__(cls, 1, 2)
        self[0].p3 = p3
        return self

After all, you already have access to the instance at creation time, you may as well initialise it there and then.

来源：https://stackoverflow.com/questions/28392810/inheritance-when-new-doesnt-return-instance-of-class