在 python 中扩展 class 的正确方法

Question

I am given a designated factory of A -type objects. I would like to make a new version of A -type objects that also have the methods in a Mixin class. For reasons that are too long to explain here, I can't use class A(Mixin) , I have to use the A_factory . Below I try to give a bare bones example.

I thought naively that it would be sufficient to inherit from Mixin to endow A -type objects with the mixin methods, but the attempts below don't work:

class A: pass

class A_factory:
    def __new__(self):
        return A()
        
class Mixin:
    def method(self):
        print('aha!')

class A_v2(Mixin):  # attempt 1
    def __new__(cls):
        return A_factory()

class A_v3(Mixin):  # attempt 2
    def __new__(cls):
        self = A_factory()
        super().__init__(self)
        return self

In fact A_v2().method() and A_v3().method() raises AttributeError: 'A' object has no attribute 'method' .

What is the correct way of using A_factory within class A_vn(Mixin) so that A -type objects created by the factory inherit the mixin methods?

Answer 1

There's no obvious reason why you should need __new__ for what you're showing here. There's a nice discussion here on the subject: Why is __init__() always called after __new__()?

If you try the below it should work:

class Mixin:
    def method(self):
        print('aha!')

class A(Mixin):
    def __init__(self):
        super().__init__()

test = A()
test.method()

If you need to use a factory method, it should be a function rather than a class. There's a very good discussion of how to use factory methods here: https://realpython.com/factory-method-python/