Format of the (string) object.

def format(self):
        """Format of the (string) object."""
        value = self._schema.get("format", None)
        if value is None:
            return
        if not isinstance(value, basestring):
            raise SchemaError(
                "format value {0!r} is not a string".format(value))
        if value in [
            'date-time',
            'regex',
        ]:
            return value
        raise NotImplementedError(
            "format value {0!r} is not supported".format(value))