继承threading.local引发的内存泄露



python的bug，直到3.2还有：

threading.local不释放成员属性，所以如果threading.local实例里引用了threading.local的实例，就不在释放了。

http://bugs.python.org/issue3757 threading.local doesn't support cyclic garbage collecting
http://bugs.python.org/issue1868 threading.local doesn't free attrs when assigning thread exits

重现例子：
#!/usr/bin/env python2.6
# coding: utf-8

from threading import local

class Client(local):

    def __init__(self):
        self.host = self
        print ('init')

    def __del__( self ):
        print ('del')

for ii in range( 3 ):
    mc = Client()
    # uncomment following line to let __del__ to be called.
    # mc.host = None