Question

This is a very contrived example as it’s not easy to explain the context in which I have ended up implementing this solution. However, if anyone can answer why this particular peculiarity happens, I’d be grateful.

The example:

class A(dict):       def __init__(self):         self['a'] = 'success'      def __getitem__(self, name):         print 'getitem'         return dict.__getitem__(name)  class B(object):     def __init__(self):         self._a = A()         setattr(self, '__getitem__', self._a.__getitem__)   b = B() c = b['a'] 

This outputs:

c = b['a'] TypeError: 'B' object is unsubscriptable 

Even though it’s a bizarre way of doing this (clearly subclassing would be more logical), why doesn’t it find the method I have explicitly set?

If I do this:

dir(b) 

I get this:

['__class__', '__delattr__', '__dict__', '__doc__', '__getattribute__', '__getitem__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__str__', '__weakref__', '_a'] 

The same issue occurs with other methods such as __iter__. What is it about explicitly defining this method that works?

Answer 1

When you use brackets [] python looks in the class. You must set the method in the class.

Here’s your code adapted:

class A(dict):       def __init__(self):         self['a'] = 'success'      def __getitem__(self, name):         print 'getitem!'         return dict.__getitem__(self, name)  class B(object):     def __init__(self):         self._a = A()         B.__getitem__ = self._a.__getitem__  b = B() c = b['a']