Hi there,

Currently the decorator._produce_input_message function uses "home-grown" introspection via f.__code__.(...) to get the parameters of the method being wrapped.
This doesn't work if the method has some other decorator, because the introspection tries to find the arguments of the decorator, instead of the decorated method.

Here's a simple example:

from functools import wraps

def deco(f):
	@wraps(f)
	def wrapped(*args, *kwargs):
		return f(*args, **kwargs)
	return wrapped

class SomeService(ServiceBase):
	@rpc(Unicode)
	@deco
	def someAction(ctx, some_argument):
		pass

Using inspect.signature(f) instead of reading f.__code__ might be a nice way to work around this problem.