mycls.py

from pluginable import Pluginable


class MyClass(Pluginable):
    pass

mycls_plugins.py

from mycls import MyClass


@MyClass.plugin
def foo(self):
    return 5

pluginable.py

from types import MethodType


class Pluginable(object):
    @classmethod
    def plugin(cls, f):
        name = f.func_name
        setattr(cls, name, MethodType(f, None, cls))

run_this.py

import mycls
import mycls_plugins


mc = mycls.MyClass()
print mc.foo() # prints 5