Background

The current implementation of UserDict attribute maps UserDict.UserDict to collections.UserDict:

MovedAttribute("UserDict", "UserDict", "collections"),

From the Python 2.7 documentation:

For backward compatibility, instances of UserDict are not iterable.

This limitation does not exist in Python3.
As a result, six introduces a bug in Python2.

Reproduction steps:

from six.moves import UserDict

d = UserDict({'key': 'output'})
for k in d:
    print(d[k])

PY3 output:

output

PY2 output:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/bxsx/.pyenv/versions/2.7.18/lib/python2.7/UserDict.py", line 40, in __getitem__
    raise KeyError(key)
KeyError: 0

Solution

This PR addresses it by updating UserDict attribute to mapUserDict.IterableUserDict in Python2 and collections.UserDict in Python3.
PR also adds moves for IterableUserDict which maps to the same attribute as UserDict.

PR fixes #281 issue