def append_form(
        self,
        obj: Union[Sequence[Tuple[str, str]], Mapping[str, str]],
        headers: Optional[MultiMapping[str]] = None,
    ) -> Payload:
        """Helper to append form urlencoded part."""
        assert isinstance(obj, (Sequence, Mapping))

        if headers is None:
            headers = CIMultiDict()

        if isinstance(obj, Mapping):
            obj = list(obj.items())
        data = urlencode(obj, doseq=True)

        return self.append_payload(
            StringPayload(
                data, headers=headers, content_type="application/x-www-form-urlencoded"
            )
        )
Helper to append form urlencoded part.
lib/aiohttp/multipart.py