Python weakref 回调和 __del__ 执行顺序

Question

在Python中，有没有办法在一个对象被销毁后调用一个函数？

我原以为使用弱引用（weakref）中的回调函数可以实现这个，但实际上，弱引用的回调是在对象被垃圾回收时调用的，但在对象的__del__方法之前。这似乎和Python文档中关于弱引用和垃圾回收的说明不太一致。下面是一个例子。

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

有没有其他常见的方法可以实现我想要的功能？我能不能在我的回调中强制执行销毁操作？

Answer 1

如果你想了解关于弱引用、__del__ 和垃圾回收的更多信息，可以查看 cpython 源代码中的 Modules/gc_weakref.txt，里面有详细的说明。

Answer 2

这里有一个解决方案，它在另一个线程上使用了一个序列化的垃圾循环。这可能是你能找到的最接近的解决办法。

import sys
from threading import Thread
from Queue import Queue
import weakref

alive = weakref.WeakValueDictionary()
dump = Queue()

def garbageloop():
    while True:
        f = dump.get()
        f()

garbage_thread = Thread(target=garbageloop)
garbage_thread.daemon = True
garbage_thread.start()

class Spam(object) :
  def __init__(self, name) :
    self.name = name
    alive[id(self)] = self

  def __del__(self) :
    sys.stdout.write("Deleting Spam:%s\n" % self.name)
    sys.stdout.flush()
    dump.put(lambda:cleaner(id(self)))

def cleaner(address):
  if address in alive:
    sys.stdout.write("Object was still alive\n")
  else:
    sys.stdout.write("Object is dead\n")
  sys.stdout.flush()

spam = Spam("first")
del spam

Answer 3

如果“随心所欲”是指“在资源离开上下文时运行代码”（而不是，比如说，“滥用垃圾回收器来做事情”），那么你可能方向搞错了。Python 把这个想法抽象成了上下文管理器，并且是和with语句一起使用的。

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!")

结果是：

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!