How to decorate class or static methods

Question

I am writing a generic class decorator which needs to apply a decorator to each method. My first approach is something like this:

def class_decorator(cls):
    for name, member in vars(cls).items():
        # Ignore anything that is not a method
        if not isinstance(member, (types.FunctionType, types.BuiltinFunctionType, classmethod, staticmethod)):
            continue

        setattr(cls, name, method_decorator(member))

    return cls

The decorator itself is not very important. Looks something like this:

def method_decorator(fn):
    @functools.wraps(fn)
    def wrapper(*args, **kwargs):
        # do something
        return fn(*args, **kwargs):

    return wrapper

Once I tested it, I ran into the problem that this does not work with static or class methods, and the following error is raised from functools.wraps:

AttributeError: 'classmethod' object has no attribute '__module__'

Yeah, classmethod or staticmethods are not normal functions, not even callables. Generally if you need to decorate a classmethod, you first apply your decorator and then the classmethod decorator, but since this is a class decorator, I cannot influence the order of the decorators.

Any good solution for this?

Topography

because the decorator creates ambiguity: the class method to bind object is not the function object (that the decorator returns), so it can't bind the method to the class – Christiano

Answer 1

After playing around for a while, I have found a solution that looks to me better than other approaches in SO. Maybe this can be helpful to somebody.

Basically the idea is the following:

• Detect members which are class or static methods
• Get the function object wrapped within these methods
• Apply the decorator to this function
• Wrap the decorated function within a classmethod or staticmethod instance
• Store it in the class again

The code looks like this:

def class_decorator(cls):
    for name, member in vars(cls).items():
        # Good old function object, just decorate it
        if isinstance(member, (types.FunctionType, types.BuiltinFunctionType)):
            setattr(cls, name, method_decorator(member))
            continue

        # Static and class methods: do the dark magic
        if isinstance(member, (classmethod, staticmethod)):
            inner_func = member.__func__
            method_type = type(member)
            decorated = method_type(method_decorator(inner_func))
            setattr(cls, name, decorated)
            continue

        # We don't care about anything else

    return cls

Topography