<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _url_format(self, service):
 """Generate URL from urlbase and service. Args: service (str):
 The endpoint service to use, i.e. gradebook Returns: str: URL to where the request should be made """

        base_service_url = '{base}{service}'.format(
            base=self.urlbase,
            service=service
        )
        return base_service_url