str currently implements some methods that UserString doesn't:

>>> set(dir(str)) - set(dir(UserString))
{'__rmod__', 'casefold', 'isprintable', 'maketrans', 'format_map', '__getnewargs__'}

casefold, isprintable & format_map (and perhaps __rmod__) should likely be available on UserString as well.

Good catch. I'm gonna look into this. Seems like you should be able to access these from UserString as well.

I have added a patch to add these to UserString. I also wrote a test case that would check the UserString, UserList, and UserDict's methods to make sure that new methods to str, list, or dict (or the removal of one of those methods from the User* version) will cause a test failure.

New changeset c06b2480766d by Raymond Hettinger in branch 'default':
Issue 22189:  Add missing methods to UserString
https://hg.python.org/cpython/rev/c06b2480766d

Thanks Joe.