def schemes(self):
        """Return supported schemes."""
        if not self._schemes:
            _schemes = []
            if self.types.keys() & _HTTP_PROTOS:
                _schemes.append('HTTP')
            if self.types.keys() & _HTTPS_PROTOS:
                _schemes.append('HTTPS')
            self._schemes = tuple(_schemes)
        return self._schemes
Return supported schemes.