`__del__`方法如果实例被存储,为什么不会被调用?

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英文:

Why is `__del__` method not called if an instance is stored?

问题

为了加深我对装饰器的理解,我尝试提出了一个替代方案,用于 Tonie Victor 的博客 中的 count_instances 装饰器(在 类装饰器的常见用例 部分)。

与其使用函数,我使用一个类作为装饰器,并扩展了功能,以便在删除装饰类的实例时保持正确的计数。为此,我装饰了装饰类的 __del__ 方法。

实际上,在我的简单测试案例中,这确实按预期工作。然而,当我尝试将装饰器从一个实例 counter 更改为包含所有实例的实例 tracker 时,它突然不再按预期工作。

更具体地说,如果我将装饰类的实例添加到 InstanceCounter 类中的列表中,那么如果删除实例,(装饰的)__del__ 方法将不再被调用,计数也将不正确。

请注意,我尚未为 tracker 实现所有功能。我计划:

  1. 用一个返回 _instances 长度的属性来替换 _count 属性;

  2. wrapper 中添加 self._instances.pop(self._instances.index(obj))

class InstanceCounter:
    def __init__(self, cls):
        self._instances = []
        self._count = 0
        if hasattr(cls, "__del__"):
            del_meth = cls.__del__
        else:
            del_meth = None
        cls.__del__ = self._del_decorator(del_meth)
        self._wrapped_cls = cls

    @property
    def instance_count(self):
        return self._count

    def __call__(self):
        self._count += 1
        obj = self._wrapped_cls()
        # self._instances.append(obj)        # 当取消注释此行时...
        return obj

    def _del_decorator(self, del_method):
        def wrapper(obj):
            self._count -= 1
            if del_method:
                del_method(obj)

        return wrapper


@InstanceCounter
class MyClass:
    def __init__(self):
        pass

    def __del__(self):                  # ... 这个方法将不会被执行。
        print(f"Deleting object of class {__class__}")


@InstanceCounter
class OtherClass:
    def __init__(self):
        pass

# 带有预期输出的测试案例。
my_a = MyClass()
print(f"{MyClass.instance_count=}")  # MyClass.instance_count=1

my_b = MyClass()
print(f"{MyClass.instance_count=}")  # MyClass.instance_count=2

del my_a
print(f"{MyClass.instance_count=}")  # MyClass.instance_count=1

oth_c = OtherClass()
print(f"{OtherClass.instance_count=}")  # OtherClass.instance_count=1

oth_d = OtherClass()
print(f"{OtherClass.instance_count=}")  # OtherClass.instance_count=2

del oth_c
print(f"{OtherClass.instance_count=}")  # OtherClass.instance_count=1

del oth_d
print(f"{OtherClass.instance_count=}")  # OtherClass.instance_count=0

为什么如果将类的实例保存在装饰类中,__del__ 方法不会被调用?如何修复这个问题?

英文:

To deepen my understanding of decorators I tried to come up with an alternative solution for the count_instances decorator in Tonie Victor's blog (section Common Use Cases of Class Decorators).

Instead of using a function, I use a class as a decorator and I extend the functionality to keep a proper count even when instances of the decorated class are deleted. For that I decorate the __del__ method of the decorated class.

In fact this works as expected on my simple test cases. However, when I try to change the decorator from an instance counter to an instance tracker that contains a list of all instances, it all of a sudden does not work as expected anymore.

More specifically, if I add the instance of the decorated class to a list in the InstanceCounter class, the (decorated) __del__ method isn't called anymore if I delete an instance and the counts are wrong.

Note that I have not implemented all functionality for the tracker yet. I planned to

  1. replace the _count attribute with a property that returns the length of _instances;

  2. add a self._instances.pop(self._instances.index(obj)) to wrapper.

class InstanceCounter:
    def __init__(self, cls):
        self._instances = []
        self._count = 0
        if hasattr(cls, "__del__"):
            del_meth = cls.__del__
        else:
            del_meth = None
        cls.__del__ = self._del_decorator(del_meth)
        self._wrapped_cls = cls

    @property
    def instance_count(self):
        return self._count

    def __call__(self):
        self._count += 1
        obj = self._wrapped_cls()
        # self._instances.append(obj)        # When this line is uncommented...
        return obj

    def _del_decorator(self, del_method):
        def wrapper(obj):
            self._count -= 1
            if del_method:
                del_method(obj)

        return wrapper


@InstanceCounter
class MyClass:
    def __init__(self):
        pass

    def __del__(self):                  # ... this method will not be executed.
        print(f"Deleting object of class {__class__}")


@InstanceCounter
class OtherClass:
    def __init__(self):
        pass

# Test cases with expected output. 
my_a = MyClass()
print(f"{MyClass.instance_count=}")  # MyClass.instance_count=1

my_b = MyClass()
print(f"{MyClass.instance_count=}")  # MyClass.instance_count=2

del my_a
print(f"{MyClass.instance_count=}")  # MyClass.instance_count=1

oth_c = OtherClass()
print(f"{OtherClass.instance_count=}")  # OtherClass.instance_count=1

oth_d = OtherClass()
print(f"{OtherClass.instance_count=}")  # OtherClass.instance_count=2

del oth_c
print(f"{OtherClass.instance_count=}")  # OtherClass.instance_count=1

del oth_d
print(f"{OtherClass.instance_count=}")  # OtherClass.instance_count=0

Why is the __del__ method not called if an instance of the class is saved in the decorating class? How can I fix this?

答案1

得分: 2

在取消注释 self._instances.append(obj) 后,my_a = MyClass() 创建了两个对该对象的引用 - my_aMyClass._instances[0]del my_a 仅删除了第一个引用,因此对象未被销毁,因此 __del__ 方法不会被调用。

如果这不是您想要的效果,您可以存储弱引用的列表而不是普通引用。要做到这一点,只需将
self._instances.append(obj) 替换为
self._instances.append(weakref.ref(obj))

英文:

After uncommenting self._instances.append(obj), my_a = MyClass() creates two references to the object - my_a and MyClass._instances[0]. del my_a deletes only the first reference, so the object is not destroyed and, thus, __del__ method is not called.

If this is not what you want, you can store a list of weak references instead of normal references. To do this, just replace
self._instances.append(obj) with
self._instances.append(weakref.ref(obj)).

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  • 本文由 发表于 2023年6月1日 03:27:41
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