Thursday, January 1, 2009

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().

3 comments:

Anonymous said...

Python 2.5.1 does not accept the metaclass= syntax in the base class description at compile time, and does not accept type as a base class at runtime (since no argument is provided to the constructor).

Benjamin said...

@Anon I said I was using 3.0 syntax; you just use the 2.x metaclass syntax for 2.x. I'm not sure why you think you can't inherit from type in 2.5, since you can...

Michael said...

Nice example thank you very much not sure I need to use it often but these quirks of python are nice to know.