unusual error in printing result

[nilamo]

       url = self.base_url + "/api/v1/person/person/?time__gt=" + since + "&time__lt=" + until

Error:
raise MissingSchema(error)
requests.exceptions.MissingSchema: Invalid URL '/api/v1/person/person/?limit=20&name__contains=&time__gt=2010-01-01&time__lt=2020-01-01&offset=20': No schema supplied.

Looks like self.base_url doesn't have a value.