>>> class A(object):
...     def __init__(self, dict_):
...         self.dict_ = dict_
...     def __getitem__(self, key):
...         return self.dict_[key]
...
>>> a = A({'foo':'bar'})
>>> a
<__main__.A object at 0x7fee5adfbe80>
>>> a['foo']
'bar'
>>> 'foo' in a
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 5, in __getitem__
KeyError: 0