这些额外的参考来自哪里？

Question

我以为我终于在python中获得了引用计数，直到我想出了下面的示例代码（在repl.it测试）。其结果提出了3个问题：

1. 为什么This和That以2个引用开始？

2. 当this被杀时，that.objects中代理引用的this对象应该会死亡;但方法仍然存在。那是为什么？

3. 删除that后，未运行gc.collect()，似乎已收集对that的引用;但对this foo方法的弱代理引用并不是。那是为什么？

提前致谢。

import sys, weakref, gc

def printf():
    print 'Actual   -> ', "This:", sys.getrefcount(This), ' ', 'That:', sys.getrefcount(That), '\n'

class This():
    def __init__(self):
        self.objects = [ self.foo ]
    def foo(self):
        pass

class That():
    def __init__(self, **kw):
        self.objects = [ weakref.proxy(kw['obj']).foo ]
        #self.objects = [ kw['obj'] ]
    def bar(self):
        pass


print '### Before ###'
print 'Expected -> ', "This: 1", ' ', 'That: 1'
printf()

this = This(); that = That(**{'obj': this})

print '### Start Runtime ###'
print 'Expected -> ', "This: 3", ' ', 'That: 2'
printf()

del this
gc.collect()

print '### Important Event ###'
print 'Expected -> ', "This: 1", ' ', 'That: 2'
printf()

del that

print '### Secondary Event ###'
print 'Expected -> ', "This: 1", ' ', 'That: 1'
printf()