为什么我不能得到预期的输出'BBB'？

Question

# -*- coding: utf-8 -*-
class tA():
    def __init__(self):
        print 'AAA'

    def __del__(self):
        print 'BBB'

class tC(tA):
    def __init__(self, a, b=0):
        tA.__init__(self)
        self.b=b             # 1. del this ok

class tD(tC):
    def __init__(self):
        a=1
        #tC.__init__(self, a)             # 2. use this ok
        tC.__init__(self, a, self.func)   # 3. use this not ok
        #tC.__init__(self, a, 3)          # 4. use this ok

    def func(self, pos):
        pass
if __name__ == '__main__':
    tD()

为什么没有“BBB”输出？你知道吗

如果我del zhe#1，输出是正常的

如果使用#2或#4，则输出正常

如果使用#3，则输出没有“BBB”？你知道吗

Answer 1

因为func是一个绑定方法，因此间接引用它绑定到的对象，这意味着您正在创建一个引用循环。你知道吗

您可以通过执行以下操作来验证这一点：

...

if __name__ == '__main__':
    import sys
    print(sys.getrefcount(tD()))

在您的案例2和案例4中应该打印1，在案例3中应该打印2。你知道吗

^{}的文档中有一个关于引用循环的注释：

[...] Some common situations that may prevent the reference count of an object from going to zero include: circular references between objects (e.g., a doubly-linked list or a tree data structure with parent and child pointers); a reference to the object on the stack frame of a function that caught an exception (the traceback stored in sys.exc_traceback keeps the stack frame alive); or a reference to the object on the stack frame that raised an unhandled exception in interactive mode (the traceback stored in sys.last_traceback keeps the stack frame alive)
[...]
Circular references which are garbage are detected when the option cycle detector is enabled (it’s on by default), but can only be cleaned up if there are no Python-level __del__() methods involved.

基本上，这意味着如果您有一个__del__方法，它将阻止清理包含引用循环的对象。你知道吗