Six: Python 2/3 compatibility helpers

Increasingly, I've seen a movement towards supporting Python 2 and Python 3 in the same code base. Having ported a few projects myself, I decided to collect the code I've duplicated between them into a library. The result is six. It includes fake byte and unicode literals, b() and u() and has wrappers for syntax changes such as print and exec. You can check out the documentation on PyPi.The license is MIT, so I hope it can see wide use in projects planning to support Python 2 and 3 simultaneously.

MRO magic

Here's some more less publicized evil you can accomplish with metaclasses. Here's a simple example file: (Note that although I'm using Python 3.0, this works with all new-style class supporting versions.)

# mromagic.py
class A(object):

    def a_method(self):
        print("A")

class B(object):

    def b_method(self):
        print("B")

class MROMagicMeta(type):

    def mro(cls):
        return (cls, B, object)

class C(A, metaclass=MROMagicMeta):

    def c_method(self):
        print("C")

Now let's play with this a little:

>>> import mromagic
>>> mycls = mromagic.C()
>>> mycls
<mromagic.C object at 0x622890>
>>> mycls.c_method()
C
>>> mycls.a_method()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'C' object has no attribute 'a_method'
>>> mycls.b_method()
B
>>> type(mycls).__mro__
(<class 'mromagic.C'>, <class 'mromagic.B'>, <class 'object'>)
>>> type(mycls).__bases__
(<class 'mromagic.A'>,)

How does this work? By overriding mro() on the a metaclass we can define a custom __mro__ for our class. Python will then traverse it instead of the default implementation, which is provided by type.mro().