_filter (Optional[str]): Document filter condition expressed in YQL.
                Defaults to None.
            yql (Optional[str]): Full YQL query to be used. Should not be specified
                if _filter or sources are specified. Defaults to None.
            kwargs (Any): Keyword arguments added to query body.
        Returns:
            VespaRetriever: Instantiated VespaRetriever.
        """
        try:
            from vespa.application import Vespa
        except ImportError:
            raise ImportError(
                "pyvespa is not installed, please install with `pip install pyvespa`"
            )
        app = Vespa(url)
        body = kwargs.copy()
        if yql and (sources or _filter):
            raise ValueError(
                "yql should only be specified if both sources and _filter are not "
                "specified."
            )
        else:
            if metadata_fields == "*":
                _fields = "*"
                body["summary"] = "short"
            else:
                _fields = ", ".join([content_field] + list(metadata_fields or []))
            _sources = ", ".join(sources) if isinstance(sources, Sequence) else "*"
            _filter = f" and {_filter}" if _filter else ""
            yql = f"select {_fields} from sources {_sources} where userQuery(){_filter}"
        body["yql"] = yql
        if k:
            body["hits"] = k
        return cls(
            app=app,
            body=body,
            content_field=content_field,
            metadata_fields=metadata_fields,
        )