Mimic Swift extension syntax in Python

extension.py

# -*- coding: utf-8 -*-
from __future__ import print_function
from functools import wraps

def extension(cls, verbose=False):
    if not isinstance(cls, type):
        raise ValueError("{} is not a class".format(cls))
        
    def func_deco(func):
        if verbose and func.__name__ in globals():
            print("Warning, overwriting {} in global scope".format(func.__name__))
        @wraps(func)
        def wrapped(*args, **kwargs):
            return func(*args, **kwargs)
        setattr(cls, func.__name__, wrapped)
        
    return func_deco

class MyClass(object):
    def __init__(self, name):
        self.name = name

@extension(MyClass)
def say(self, msg):
    print("{} says {}".format(self.name, msg))