Retrieve Import string from a decorated classmethod

Question

I am trying to retrieve the import-string in a decorated @classmethod to register this string in an pipeline system. But when I'm inspecting the function object in the decorator function a cannot find any information about the class object or the class name.

So the code looks something like this:

def import_string_decorator(**opt_kwargs):

    def wrap(f):

        # Here is the problem
        if inspect.ismethod(f):
            class_name = f.im_self.__name__
            import_string = f.__module__ + "." class_name + "." + f.__name__
            # But this doesn't work because f no longer is a <bound method to class 'SomeClass'> but is a regular <function>
        else:
            import_string = f.__module__ + "." + f.__name__

        # Register the string
        do_something(import_string, **opt_kwargs)

        def wrapped_f(*args, **kwargs):

            f(*args, **kwargs)

        return wrapped_f

    return wrap


# Decorated Class
opt_dict = {"some": "values"}

class SomeClass(object):

    @classmethod
    @import_string_decorator(**opt_dict)
    def double_decorated_function(cls, *args, **kwargs):

        pass

But I haven't found a way to retrieve the class object of the decorated function. The inspect.ismethod() function also returns Falsebecause it checks isinstance(types.MethodType) underneath.

Answer 1

What you want cannot be done with function decorators. The function object is created and decorated before the class object is built. Python executes the class body first, and the resulting names then form the class attributes.

Binding of methods then takes place dynamically, as you access the name as an attribute, using the descriptor protocol.

You need to hook into class creation for you to get access to the class name; you can use a class decorator, or use a metaclass. You can combine these techniques with function decorators if that makes it easier:

@registered_class
class SomeClass(object):

    @classmethod
    @import_string_decorator(**opt_dict)
    def double_decorated_function(cls, *args, **kwargs):
        pass

where the import_string_decorator could annotate the function (you can set an attribute on it, for example) for the registered_class decorator to inspect when the class is being decorated.