使用元类跟踪 python 中的实例

Question

我需要跟踪一些类的实例（并用这些类做其他事情）。我不想在有问题的类中声明任何额外的代码，因此最好在元类中处理所有内容。

我想不通的是如何为那些类的每个新实例添加弱引用。例如：

class Parallelizable(type):
    def __new__(cls, name, bases, attr):
        meta = super().__new__(cls, name, bases, attr)
        # storing the instances in this WeakSet
        meta._instances = weakref.WeakSet()
        return meta

    @property
    def instances(cls):
        return [x for x in cls._instances]

class Foo(metaclass=Parallelizable)
    def __init__(self, name):
        super().__init__()
        self.name = name

        # I would like to avoid having to do that - instead have the metaclass manage it somehow
        self._instances.add(self)

有什么想法吗？我似乎无法在元类端找到一个钩子来进入 Foo 的 __init__....

Answer 1

您可以在 Parallizable.__new__ 中修饰 attrs['__init__'] 方法：

import weakref
import functools
class Parallelizable(type):
    def __new__(meta, name, bases, attrs):
        attrs['__init__'] = Parallelizable.register(attrs['__init__'])
        cls = super().__new__(meta, name, bases, attrs)
        cls._instances = weakref.WeakSet()
        return cls

    @classmethod
    def register(cls, method):
        @functools.wraps(method)
        def newmethod(self, *args, **kwargs):
            method(self, *args, **kwargs)
            self._instances.add(self)
        return newmethod

    @property
    def instances(cls):
        return [x for x in cls._instances]

class Foo(metaclass=Parallelizable):
    def __init__(self, name):
        "Foo.__init__ doc string"
        super().__init__()
        self.name = name

# Notice that Foo.__init__'s docstring is preserved even though the method has been decorated
help(Foo.__init__)
# Help on function __init__ in module __main__:
#
# __init__(self, name)
#     Foo.__init__ doc string

stilton = Foo('Stilton')
gruyere = Foo('Gruyere')
print([inst.name for inst in Foo.instances])
# ['Gruyere', 'Stilton']

del stilton
print([inst.name for inst in Foo.instances])
# ['Gruyere']

Answer 2

metaclass 上的方法在其 "afiliated" classes 的每个新实例为 __call__ 时调用。如果你把记录实例的代码放在那里，这就是你需要的所有工作：


from weakref import WeakSet

# A convenient class-level descriptor to retrieve the instances:

class Instances:
    def __get__(self, instance, cls):
        return [x for x in cls._instances]

class Parallelizable(type):
    def __init__(cls, name, bases, attrs, **kw):
        super().__init__(name, bases, attrs, **kw)
        cls._instances = WeakSet()
        cls.instances = Instances()

    def __call__(cls, *args, **kw):
        instance = super().__call__(*args, **kw)
        cls._instances.add(instance)
        return instance

同样的代码在没有描述符的情况下也能工作——这是一个很好的方法来拥有一个 class 属性来报告实例。但是如果 WeakSet 足够了，这个代码就足够了：


from weakref import WeakSet
class Parallelizable(type):
    def __init__(cls, name, bases, attrs, **kw):
        super().__init__(name, bases, attrs, **kw)
        cls.instances = WeakSet()

    def __call__(cls, *args, **kw):
        instance = super().__call__(*args, **kw)
        cls.instances.add(instance)
        return instance

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