python/cpython · error · RuntimeError

unable to write; sendfile is in progress

Error message

unable to write; sendfile is in progress

What it means

Error "unable to write; sendfile is in progress" thrown in python/cpython.

Source

Thrown at Lib/asyncio/proactor_events.py:343

            if length > -1:
                self._data_received(data, length)


class _ProactorBaseWritePipeTransport(_ProactorBasePipeTransport,
                                      transports.WriteTransport):
    """Transport for write pipes."""

    _start_tls_compatible = True

    def write(self, data):
        if not isinstance(data, (bytes, bytearray, memoryview)):
            raise TypeError(
                f"data argument must be a bytes-like object, "
                f"not {type(data).__name__}")
        if self._eof_written:
            raise RuntimeError('write_eof() already called')
        if self._empty_waiter is not None:
            raise RuntimeError('unable to write; sendfile is in progress')

        if not data:
            return

        if self._conn_lost:
            if self._conn_lost >= constants.LOG_THRESHOLD_FOR_CONNLOST_WRITES:
                logger.warning('socket.send() raised exception.')
            self._conn_lost += 1
            return

        # Observable states:
        # 1. IDLE: _write_fut and _buffer both None
        # 2. WRITING: _write_fut set; _buffer None
        # 3. BACKED UP: _write_fut set; _buffer a bytearray
        # We always copy the data, so the caller can't modify it
        # while we're still waiting for the I/O to happen.
        if self._write_fut is None:  # IDLE -> WRITING
            assert self._buffer is None

View on GitHub (pinned to bc6749cc3b)

Solutions

1. Wait for the sendfile operation to finish before writing; chain the write after the sendfile future completes.
2. Do not interleave transport.write() with loop.sendfile() on the same transport.

When it happens

Trigger: Raised when write() is attempted while a sendfile operation is in progress on the transport.

Common situations: See trigger scenarios.

AI-assisted analysis of python/cpython@bc6749cc3b (2026-08-14). Data as JSON: /api/errors/8559f12dd0c2db18. Report an issue: GitHub.