如何跟踪传递给元类的关键字? [英] How to keep track of keywords passed to metaclass?

问题描述

我有一个接受关键字参数的元类:

I have a metaclass that accepts keyword arguments:

class M(type):
    def __new__(cls, *args, **kwargs):
        print(*kwargs.items())
        super().__new__(cls, *args)

它按预期工作:

class A(metaclass=M, test='X'): pass

产生对象A和打印输出

('test', 'X')

我想使用此之类的东西来复制A:

I would like to make a copy of A using something like this:

def copy_class(cls, name=None):
    if name is None:
        name = cls.__name__
    return type(cls)(name, cls.__bases__, dict(cls.__dict__))

A_copy = copy_class(A, 'A_copy')

但是，关键字不存在.当type(cls) 需要附加参数，或者在不存在附加副作用时会产生不同的副作用时，这尤其是个问题.

However, the keywords are not present. This is especially a problem when type(cls) requires additional arguments or otherwise produces different side-effects when they are not there.

我知道我可以将M关键字存储在类对象中的某个位置，但这不是一个很好的解决方案，因为那时我的复制方法非常依赖于类.

I am aware that I can have M stash the keywords in the class object somewhere, but this is not a very good solution because then my copy method is very class-dependent.

是否有内置的方法来检索使用Python创建类的关键字?

Is there a built-in way to retrieve the keywords a class was created with in Python?

推荐答案

Python不会为您保存关键字参数.这很容易演示:

Python will not save the keyword arguments for you. This is simple to demonstrate:

>>> class DoesntSaveKeywords(type):
...     def __new__(cls, name, bases, dict, **kwargs):
...         return super().__new__(cls, name, bases, dict)
... 
>>> class PrintsOnDel:
...     def __del__(self):
...         print('__del__')
... 
>>> class Foo(metaclass=DoesntSaveKeywords, keyword=PrintsOnDel()):
...     pass
... 
__del__

此外，我认为尝试复制这样的类的基本想法没有道理.不能保证该类的__dict__看起来像最初传递给元类构造函数的东西，或者再次调用该元类构造函数将做任何合理的事情.

Also, I don't think the underlying idea of trying to copy a class like this makes sense. There's no guarantee that the class's __dict__ looks anything like what was originally passed to the metaclass constructor, or that calling the metaclass constructor again will do anything remotely reasonable.

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