What causes "Cannot call write() after write_eof()" in aio-libs/aiohttp?

Guard writes with if not resp.prepared or resp._eof_sent — better, track your own 'done' flag.

aio-libs/aiohttp · error · RuntimeError

Cannot call write() after write_eof()

Error message

Cannot call write() after write_eof()

What it means

Once write_eof() has been called the response is finalized and the writer is torn down (self._payload_writer set to None). Calling write() afterwards is illegal because the body is already terminated; write() checks self._eof_sent at line 456 first.

Source

Thrown at aiohttp/web_response.py:457

        assert writer is not None
        # status line
        version = request.version
        status_line = f"HTTP/{version[0]}.{version[1]} {self._status} {self._reason}"
        await writer.write_headers(status_line, self._headers)

        # Send headers immediately if not opted into buffering
        if self._send_headers_immediately:
            writer.send_headers()

    async def write(
        self, data: Union[bytes, bytearray, "memoryview[int]", "memoryview[bytes]"]
    ) -> None:
        assert isinstance(
            data, (bytes, bytearray, memoryview)
        ), "data argument must be byte-ish (%r)" % type(data)

        if self._eof_sent:
            raise RuntimeError("Cannot call write() after write_eof()")
        if self._payload_writer is None:
            raise RuntimeError("Cannot call write() before prepare()")

        await self._payload_writer.write(data)

    async def drain(self) -> None:
        assert not self._eof_sent, "EOF has already been sent"
        assert self._payload_writer is not None, "Response has not been started"
        warnings.warn(
            "drain method is deprecated, use await resp.write()",
            DeprecationWarning,
            stacklevel=2,
        )
        await self._payload_writer.drain()

    async def write_eof(self, data: bytes = b"") -> None:
        assert isinstance(
            data, (bytes, bytearray, memoryview)

View on GitHub (pinned to c0ef574e29)

Solutions

Guard writes with if not resp.prepared or resp._eof_sent — better, track your own 'done' flag.
Move streaming into the handler coroutine and return only after all writes complete.
Cancel/clean up background tasks before the handler returns.

Example fix

# before
await resp.write_eof()
await resp.write(b'more')  # raises RuntimeError

# after
await resp.write(b'more')
await resp.write_eof()  # finalize once, last

Defensive patterns

Strategy: validation

Validate before calling

async def safe_write(resp, data):
    if resp._eof_sent:
        return  # response already finalized
    await resp.write(data)

Try / catch

try:
    await resp.write(data)
except RuntimeError as e:
    if 'after write_eof' in str(e):
        return  # ignore late writes from background tasks

Prevention

Centralize all writes in the handler coroutine and return only when done.
Track your own 'finalized' flag instead of relying on internals.
Cancel background tasks before the handler returns.

When it happens

Trigger: Calling await resp.write(data) after await resp.write_eof(), or after the framework has auto-finalized the response (e.g. returning from the handler after a manual write_eof, or a background task writing to an already-closed response).

Common situations: A cleanup/background coroutine writing to a response whose handler already returned; double-finalization; fire-and-forget tasks that outlive the request.

Related errors

AI-assisted analysis of aio-libs/aiohttp@c0ef574e29 (2026-08-04). Data as JSON: /data/errors/721d7622bbd1a9e6.json. Report an issue: GitHub.