2

假设我有以下类，其中既有deleter属性的客户，也有实例本身：

class Hello:
    def __init__(self):
        self._is_open = None
    @property
    def is_open(self):
        return self._is_open
    @is_open.deleter
    def is_open(self):
        print("Using custom deleter!")
        del self._is_open
    def __delattr__(self, attr):
        print ('Deleting attr %s' % attr)
        super().__delattr__(attr)

并称之为：

>>> hello = Hello()
>>> del hello.is_open
Deleting attr is_open
Using custom deleter!
Deleting attr _is_open

看起来它首先调用__delattr__on is_open，然后调用@is_open.deleter，然后调用__delattr__on _is_open。为什么删除者的事件链会这样工作？

4

1 回答 1

2

Python属性是描述符。它们是通过描述符协议实现的。

数据模型挂钩__delattr__优先于描述符协议。因此，如果您定义了自定义__delattr__方法，则将优先调用该方法而不是属性删除器。

事实上，它的默认实现__delattr__将在必要时调用描述符，您可以通过注释掉开头的行来验证这一点super（您应该看到现在根本不会调用属性删除器）。

有了这个推理，你就可以理解这样的事件链：

Deleting attr is_open
# the del statement `del hello.is_open` is directly invoking Hello.__delattr__,
# passing in attr="is_open" as argument

# Now, the implementation of `Hello.__delattr__` calls
# `super().__delattr__(attr)`, passing along the argument attr="is_open", which
# then invokes a descriptor for that attribute (i.e. the function
# `Hello.is_open.fdel` is about to be called)

Using custom deleter!
# This comes from within the property (`Hello.is_open`) deleter.

Deleting attr _is_open
# The implementation of the `is_open` deleter actually uses another del
# statement, i.e. `del self._is_open`. This invokes again `Hello.__delattr__`,
# passing attr="_is_open" as an argument. However, this time there is no
# descriptor with the name `_is_open` present so an attribute gets deleted from
# the instance namespace instead. Note that the attribute `self._is_open` was
# there in `self.__dict__` already because it gets created during the __init__
# method when `hello = Hello()` is executed.

重要的是要注意__delattr__第一次和第二次收到不同的论点："is_open"第一次然后"_is_open"第二次。

于 2019-10-16T15:28:11.280 回答