Using a class decorator for applying a metaclass in both Python 2 and 3
When you want to create a class including a metaclass, making it compatible with both Python 2 and 3 can be a little tricky.
from six import with_metaclass class Meta(type): pass class Base(object): pass class MyClass(with_metaclass(Meta, Base)): pass
The basic trick is that you can call any metaclass to produce a class for you, given a name, a sequence of baseclasses and the class body.
six produces a new, intermediary base class for you:
>>> type(MyClass) <class '__main__.Meta'> >>> MyClass.__mro__ (<class '__main__.MyClass'>, <class 'six.NewBase'>, <class '__main__.Base'>, <type 'object'>)
This can complicate your code as for some usecases you now have to account for the extra
six.NewBase baseclass present.
Rather than creating a base class, I’ve come up with a class decorator that replaces any class with one produced from the metaclass, instead:
def with_metaclass(mcls): def decorator(cls): body = vars(cls).copy() # clean out class body body.pop('__dict__', None) body.pop('__weakref__', None) return mcls(cls.__name__, cls.__bases__, body) return decorator
which you’d use as:
class Meta(type): pass class Base(object): pass @with_metaclass(Meta) class MyClass(Base): pass
which results in a cleaner MRO:
>>> type(MyClass) <class '__main__.Meta'> >>> MyClass.__mro__ (<class '__main__.MyClass'>, <class '__main__.Base'>, <type 'object'>)
As it turns out, Jason Coombs took Guido’s time machine and added the same functionality to the
six library last summer. Not only that, he included support for classes with
__slots__ in his version. Thanks to Mikhail Korobov for pointing this out.
six decorator is called
@six.add_metaclass(Meta) class MyClass(Base): pass
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