<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def set_url(self, url = None):
 """This method is used to set the url. url must be a string. """

                
        if url is None or type(url) is not str:
            raise KPError("Need a new image number")
        else:
            self.url = url
            self.last_mod = datetime.now().replace(microsecond=0)
            return True