Memoized objects still have their __init__() invoked?

问题

So I am creating a memoized class, and have been observing a strange behavior.

Here's the snippet of the code:

class SomeClass(object):

    _Memoized = {}

    def __new__(cls, id: str, *args, **kwargs):
        if id not in cls._Memoized:
            print('New Instance')
            cls._Memoized[id] = super(SomeClass, cls).__new__(cls, *args, **kwargs)
        else:
            print('Existing Instance')
        return cls._Memoized[id]

    def __init__(self, id: str):
        print('Running init')
        self.id = id


def test():
    w1 = SomeClass(id='w1')
    wx = SomeClass(id='w1')
    print(id(w1), id(wx), id(w1) == id(wx))

test()

Running the above code results in:

New Instance
Running init
Existing Instance
Running init   <===-------------------???
140008534476784 140008534476784 True

My questions: During the second invocation of SomeClass(), why does it execute the __init__ constructor? Wasn't the __init__ constructor only invoked on instantiating? Is there a way to prevent the __init__ from being invoked?

回答1:

The purpose of __new__ is to create a new instance, which is why Python calls __init__ on it. You might instead override __call__ on a metaclass to avoid creating a new instance.

class MemoMeta(type):
    def __init__(self, name, bases, namespace):
        super().__init__(name, bases, namespace)
        self.cache = {}
    def __call__(self, id_, *args, **kwargs):
        if id_ not in self.cache:
            print('New Instance')
            self.cache[id_] = super().__call__(id_, *args, **kwargs)
        else:
            print('Existing Instance')
        return self.cache[id_]

class SomeClass(metaclass=MemoMeta):
    def __init__(self, id_, *args, **kwargs):
        print('Running init')
        self.id = id_


def test():
    w1 = SomeClass(id_='w1')
    wx = SomeClass(id_='w1')
    print(id(w1), id(wx), id(w1) == id(wx))

test()

来源：https://stackoverflow.com/questions/47785795/memoized-objects-still-have-their-init-invoked