#!/usr/bin/env python
"""
meta2and3.py

Describes a simple approach to employing metaclasses in a manner simultaneously
compatible with both Python 2 and 3.
"""

# The actual definition of the metaclass remains the same in both Python 2 and 3
class SillyWalksMetaClass(type):
    """
    A metaclass describing a class that acts as a singleton and provides a
    registry of funny walking styles, something no Monty Python fan should
    be without.
    """

    def __init__(self, class_name, bases, namespace):

        if not hasattr(self, 'walk_types'):
            self.walk_types = []
        else:
            self.walk_types.append(self)

    def __str__(self):
        return self.__name__

    def get_walks(self, *args, **kwargs):
        """Returns a list ordered by the class name"""
        return sorted(self.walk_types, key=lambda x: x.__name__)

# Its the *use* of the metaclass which changes in 3. Never fear,
# Knights who say Ni! are here...

# in Python 2.x you'd create your metaclass derived class as:
#
#   class SillyWalk(object):
#       """
#       Subclasses implement a silly walk of stunning sillyness
#       """
#
#       __metaclass__ = SillyWalksMetaClass


# As of Python 3 the __metaclass__ attribute means nothing; instead you
# supply the class definition with a keyword:
#
#   class SillyWalk(metaclass=SillyWalksMetaClass):
#       """
#       Subclasses implement a silly walk of stunning sillyness
#       """


# AND NOW FOR SOMETHING COMPLETELY DIFFERENT...
# But what we need is a method which works for both, since
# Python 2.x will raise a SyntaxError which can't be hidden
# behind an if or try block. The 2 *and* 3 solution is:

SillyWalk = SillyWalksMetaClass('SillyWalk', (object, ), {})

# As you see, it was only a flesh wound. We have a simple
# and clean approach for metaclasses for both Python 2 and
# 3.

# Subclassing remains the same for Python 2.x and Python 3+,
# lets create four favorite silly walks:

class Skip(SillyWalk):
    pass

class LurchAndSkip(SillyWalk):
    pass

class CleeseSpecial(SillyWalk):
    'The quintessential silly walk'

class HopWeaveLurchShudder(SillyWalk):
    pass

# got all four?
assert len(SillyWalk.get_walks()) == 4
# right then, lets have a look
print(', '.join([str(walk) for walk in SillyWalk.get_walks()]))
# Being a singleton, you'd expect the following to be identical:
assert SillyWalk.get_walks() == HopWeaveLurchShudder.get_walks()
# and it is...

# $ python3.0 meta2and3.py
# CleeseSpecial, HopWeaveLurchShudder, LurchAndSkip, Skip
# $ python2.6 meta2and3.py
# CleeseSpecial, HopWeaveLurchShudder, LurchAndSkip, Skip
# $ python2.5 meta2and3.py
# CleeseSpecial, HopWeaveLurchShudder, LurchAndSkip, Skip