python：无法解释的无限递归与__repr__

Question

这是一段代码，它进入一个无限递归循环，它只包含__repr__函数，似乎在调用自身。但我真的无法看到，它是如何自称的。而且，我甚至无法理解，它是如何被称为的：

class MyList(list): #this is storage for MyDict objects
    def __init__(self):
        super(MyList, self).__init__()

class MyDict(dict):
    def __init__(self, mylist):
        self.mylist = mylist #mydict remembers mylist, to which it belongs
    def __hash__(self):
        return id(self)
    def __eq__(self, other):
        return self is other
    def __repr__(self):
        return str(self.mylist.index(self)) #!!!this is the crazy repr, going into recursion
    def __str__(self):
        return str(self.__repr__())

mylist = MyList()
mydict = MyDict(mylist)
mydict.update({1:2})
print str(mylist.index(mydict)) #here we die :(

执行此代码会导致：

Traceback (most recent call last):
  File "test_analogue.py", line 20, in <module>
    print str(mylist.index(mydict))
  File "test_analogue.py", line 13, in __repr__
    return str(self.mylist.index(self))
  File "test_analogue.py", line 13, in __repr__
  ... 
  ... (repetition of last 2 lines for ~666 times)
  ...
  File "test_analogue.py", line 13, in __repr__
    return str(self.mylist.index(self))
  RuntimeError: maximum recursion depth exceeded while calling a Python object

您了解str(mylist.index(mydict))如何设法致电__repr__？我完全不解。谢谢！

Answer 1

>> mylist.index('foo')
ValueError: 'foo' is not in list

您实际上从未将mydict添加到mylist，因此index方法会尝试引发此错误。该错误包含dict的repr。当然，dict的repr试图在列表中查找它不在的index，这引发了一个异常，其错误消息是使用dict的repr计算的，当然，试着在列表中查找它不在的index，并且......