为什么Django的信号处理默认使用弱引用进行回调?

信号处理程序被存储为弱引用,以避免它们引用的对象被垃圾收集(例如,在显式删除信号处理程序之后),这仅仅是因为信号仍在四处传播。

绑定方法保留对其所属对象的引用(否则,它们无法填充 self 参见 Python documentation )请考虑以下代码:

import gc
class SomeLargeObject(object):
    def on_foo(self): pass

slo = SomeLargeObject()
callbacks = [slo.on_foo]

print [o for o in gc.get_objects() if isinstance(o, SomeLargeObject)]
del slo
print [o for o in gc.get_objects() if isinstance(o, SomeLargeObject)]
callbacks = []
print [o for o in gc.get_objects() if isinstance(o, SomeLargeObject)]

输出:

[<__main__.SomeLargeObject object at 0x15001d0>]
[<__main__.SomeLargeObject object at 0x15001d0>]
[]

在回调上保留weakerfs时,需要知道的一件重要事情是,您不能直接weakeref绑定的方法,因为它们总是动态创建的:

>>> class SomeLargeObject(object):
...  def on_foo(self): pass
>>> import weakref
>>> def report(o):
...  print "about to collect"
>>> slo = SomeLargeObject()
>>> #second argument: function that is called when weakref'ed object is finalized
>>> weakref.proxy(slo.on_foo, report)
about to collect
<weakproxy at 0x7f9abd3be208 to NoneType at 0x72ecc0>