Replaced docstring with comments.  Prevents subclass contamination.
Added the missing __cmp__() method and a test for __cmp__().
Used try/except style in preference to has_key() followed by a look-up.
Used iteritem() where possible to save creating a long key list and
   to save redundant lookups.
Expanded .update() to look for the most helpful methods first and gradually
   work down to a mininum expected interface.
Expanded documentation to be more clear on how to use the class.

and add a mixin to UserDict.py to make it easier to implement a full
dictionary interface.

and suggeest that new code that does not require compatibility with older
Python versions subclass dictionary, list, or str.

<lannert@lannert.rz.uni-duesseldorf.de>.

than the built-in string types (suggested by Moshe Zadka
<moshez@math.huji.ac.il>).

Clarified what "can be converted to a string" means.

Fixed a few markup nits.