UserDict does not directly support iteration:

>>> import UserDict
>>> ud = UserDict.UserDict()
>>> ud['a'] = 1
>>> [k for k in ud]
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "C:\Python24\lib\UserDict.py", line 17, in
__getitem__
    def __getitem__(self, key): return self.data[key]
KeyError: 0

The fix is to define __iter__ = iterkeys:
>>> class UD(UserDict.UserDict):
...   __iter__ = UserDict.UserDict.iterkeys
...
>>> ud = UD()
>>> ud['a'] = 1
>>> [k for k in ud]
['a']

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user_id=80475

This could not be done for reasons of backwards compatability.

To get what you want, use UserDict.IterableUserDict.