Propagating class decorators to inherited classes

Question

import inspect
import functools

def for_all_test_methods(decorator):
    def decorate(cls):
        for name, value in inspect.getmembers(cls, inspect.isroutine):
            if name.startswith('test'):
                setattr(cls, name, test_decorator(getattr(cls, name)))
        return cls
    return decorate

def test_decorator(func):
    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        print(func.__name__, args, kwargs)
        res = func(*args, **kwargs)
        return res
    return wrapper

@for_all_test_methods(test_decorator)
class Potato(object):
    def test_method(self):
        print('in method')

class Spud(Potato):
    def test_derived(self):
        print('in derived')

Now if I create a spud instance the test_method which it has inherited remains decorated, but it has an undecorated method test_derived. Unfortunately, if I add the class decorator onto Spud aswell, then his test_method gets decorated twice!

How do I correctly propagate decorators from the parent class onto the children?

Answer 1

Here is how you can accomplish this by using a metaclass instead of decorating the class:

import inspect
import functools

def test_decorator(func):
    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        print(func.__name__, args, kwargs)
        res = func(*args, **kwargs)
        return res
    return wrapper

def make_test_deco_type(decorator):
    class TestDecoType(type):
        def __new__(cls, clsname, bases, dct):
            for name, value in dct.items():
                if name.startswith('test') and inspect.isroutine(value):
                    dct[name] = decorator(value)
            return super().__new__(cls, clsname, bases, dct)
    return TestDecoType

class Potato(object, metaclass=make_test_deco_type(test_decorator)):
    def test_method(self):
        print('in method')

class Spud(Potato):
    def test_derived(self):
        print('in derived')

On Python 2.x you would use __metaclass__ = make_test_deco_type(test_decorator) as the first line of the class body instead of having the metaclass=... portion of the class statement. You would also need to replace super() with super(TestDecoType, cls).