PEP8 horizon cleanup

The None, True, and False values are singletons.

All variable *comparisons* to singletons should use 'is' or 'is not'.
All variable *evaluations* to boolean should use 'if' or 'if not'.
All Object type comparisons should use isinstance() instead of comparing types directly

Programming Recommendations
    - Comparisons to singletons like None should always be done with
      'is' or 'is not', never the equality operators.

    - Don't compare boolean values to True or False using ==
        Yes: if greeting:
        No: if greeting == True:
        Worse: if greeting is True:

    - Object type comparisons should always use isinstance() instead
      of comparing types directly.

        Yes: if isinstance(obj, int):

        No: if type(obj) is type(1):

      When checking if an object is a string, keep in mind that it might be a
      unicode string too! In Python 2.3, str and unicode have a common base
      class, basestring, so you can do:

        if isinstance(obj, basestring):