记忆对象仍然调用了它们的__init __()吗?

问题描述

所以我正在创建一个记忆类，并且一直观察到一种奇怪的行为.

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

这是代码段:

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()

运行上面的代码将导致:

Running the above code results in:

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

我的问题:在SomeClass()的第二次调用期间，为什么要执行__init__构造函数? __init__构造函数不是仅在实例化时调用吗?有没有办法防止__init__被调用?

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?

推荐答案

__new__的目的是创建一个新实例，这就是Python在其上调用__init__的原因.您可以改用元类上的__call__以避免创建新实例.

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()

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