# packaging/metadata.py

class Metadata:
    # [...]
    
    def set(self, name, value):
        """Control then set a metadata field."""

        # [...]

        # FIXME this rejects UNKNOWN, is that right?
        elif name in _VERSIONS_FIELDS and value is not None:
            if not is_valid_versions(value):
                logger.warning('%r: %r is not a valid version (field %r)',
                               project_name, value, name)
        elif name in _VERSION_FIELDS and value is not None:
            if not is_valid_version(value):
                logger.warning('%r: %r is not a valid version (field %r)',
                               project_name, value, name)