async def write_headers(
        self, status_line: str, headers: "CIMultiDict[str]"
    ) -> None:
        """Write request/response status and headers."""
        if self._on_headers_sent is not None:
            await self._on_headers_sent(headers)

        # status + headers
        buf = _serialize_headers(status_line, headers)
        self._write(buf)
Write request/response status and headers.
lib/aiohttp/http_writer.py