printable_func.py

def printable_func(str_func=None):
    if str_func is None:
        str_func = lambda func: func.__name__

    def inner(func):
        return type(
            func.__name__, (object, ),
            {
                **vars(func),
                '__call__': 
                    lambda self, *args, **kwargs: func(*args, **kwargs),
                '__str__':  
                    lambda self: str_func(func),
            }
        )()
    
    return inner
    

def main():
    def boring_spam():
        return 'spam'

    @printable_func()
    def printable_spam():
        return 'spam'

    print(boring_spam())        # spam
    print(boring_spam)          # <function boring_spam at ...>

    print(printable_spam())     # spam
    print(printable_spam)       # printable_spam


if __name__ == '__main__':
    main()