在局部变量上使用显式del在python

问题描述

在python中使用显式

使用 del

code>是确保
a对象的 __ del __ 方法被调用的唯一方法

这是非常重要的。为了帮助，让我们使用 __ del __ 创建一个类，并检查它是否被调用：

 >>>> class visdel（object）：
 ... def __del __（self）：print'del'，id（self）
 ... 
>>> d = visdel（）
>>>> a = list（）
>>>> a.append（d）
>>>> del d 
>>>> 
  

del 不会确保 __ del __ 被调用： del 删除一个引用，只有删除最后引用才会调用 __ del __ 。所以，也：

 >>> a.append（visdel（））
>>> a [：] = [1，2，3] 
 del 550864 
 del 551184 
  

当最后一个引用消失时（包括不涉及 del 的方式，例如这种情况下的切片分配或其他名称重新绑定，其他插槽），然后 __ del __ 被调用 - 是否 del

因此，除非你特别需要清理命名空间（通常是模块的名称空间）命名空间，但可以想象一个类或实例），因为某些特定的原因，不要打扰 del （它可以偶尔方便地从容器中删除项目，我发现我经常使用容器的 pop 方法或项目或切片分配，即使 ！ - ）。

What are the best practices and recommendations for using explicit del statement in python? I understand that it is used to remove attributes or dictionary/list elements and so on, but sometimes I see it used on local variables in code like this:

def action(x):
    result = None
    something = produce_something(x)
    if something:
        qux = foo(something)
        result = bar(qux, something)
        del qux
    del something
    return result

Are there any serious reasons for writing code like this?

Edit: consider qux and something to be something "simple" without a __del__ method.

解决方案

I don't remember when I last used del -- the need for it is rare indeed, and typically limited to such tasks as cleaning up a module's namespace after a needed import or the like.

In particular, it's not true, as another (now-deleted) answer claimed, that

and it's very important to understand this. To help, let's make a class with a __del__ and check when it is called:

>>> class visdel(object):
...   def __del__(self): print 'del', id(self)
... 
>>> d = visdel()
>>> a = list()
>>> a.append(d)
>>> del d
>>>

See? del doesn't "make sure" that __del__ gets called: del removes one reference, and only the removal of the last reference causes __del__ to be called. So, also:

>>> a.append(visdel())
>>> a[:]=[1, 2, 3]
del 550864
del 551184

when the last reference does go away (including in ways that don't involve del, such as a slice assignment as in this case, or other rebindings of names and other slots), then __del__ gets called -- whether del was ever involved in reducing the object's references, or not, makes absolutely no difference whatsoever.

So, unless you specifically need to clean up a namespace (typically a module's namespace, but conceivably that of a class or instance) for some specific reason, don't bother with del (it can be occasionally handy for removing an item from a container, but I've found that I'm often using the container's pop method or item or slice assignment even for that!-).

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