To perhaps shed a little light on the None thing and why it would be useful, consider the following:

    def munge(data, convert=None):
        data = map(convert, data)
        ...

ie. treating None as identity gives you sane default behaviour in the case that you want to optionally provide a mapping functionality. Granted, you can easily do an "if convert is not None" but I think this is slightly nicer.

Welp, add it to the list of things I think are better in py2 than py3.