Python weakref 回调和 __del__ 执行顺序

Question

在 Python 中，有没有办法在对象完成后调用函数？

我认为 weakref 中的回调会执行此操作，但似乎一旦对象被垃圾回收，但在调用对象 __del__ 方法之前调用 weakref 的回调。这似乎与the notes on weakrefs and garbage collection in the Python trunk相反.这是一个例子。

import sys
import weakref

class Spam(object) :
  def __init__(self, name) :
    self.name = name

  def __del__(self) :
    sys.stdout.write("Deleting Spam:%s\n" % self.name)
    sys.stdout.flush()

def cleaner(reference) :
  sys.stdout.write("In callback with reference %s\n" % reference)
  sys.stdout.flush()

spam = Spam("first")
wk_spam = weakref.ref(spam, cleaner)
del spam

我得到的输出是

$ python weakref_test.py 
In callback with reference <weakref at 0xc760a8; dead>
Deleting Spam:first

还有其他一些常规方法可以做我想做的事吗？我能否以某种方式在我的回调中强制完成？

Answer 1

如果“做你想做的事”意味着“当资源离开上下文时运行代码”(而不是，例如，“滥用垃圾收集器做事”)，那么你的方向是错误的。 Python 将整个想法提取为 context-managers , 与 with 语句一起使用。

from __future__ import with_statement
import sys
class Spam(object):
    def __init__(self, name):
        self.name = name

    def __enter__(self):
        sys.stdout.write("Entering Spam:%s\n" % self.name)
        sys.stdout.flush()

    def __exit__(self, type, value, traceback):
        sys.stdout.write("Lets clean up Spam:%s\n" % self.name) 
        if type is None:
            sys.stdout.write("Leaving Spam:%s in peace\n" % self.name)
            return
        else:
            sys.stdout.write("Leaving Spam:%s with Exception:%r\n" % (self.name, value))


with Spam("first") as spam:
    pass

with Spam("2nd") as spam:
    raise Exception("Oh No!")

给出:

Entering Spam:first
Lets clean up Spam:first
Leaving Spam:first in peace
Entering Spam:2nd
Lets clean up Spam:2nd
Leaving Spam:2nd with Exception:Exception('Oh No!',)
Traceback (most recent call last):
  File "asd.py", line 24, in <module>
    raise Exception("Oh No!")
Exception: Oh No!