python/cpython · error · TypeError

data argument must be a bytes, bytearray, or memoryview obje

Error message

data argument must be a bytes, bytearray, or memoryview object, not {type(data).__name__!r}

What it means

Raised by _SelectorSocketTransport.write() as TypeError when the data argument is not an instance of bytes, bytearray, or memoryview. Transports move raw bytes only; unlike sockets or some third-party APIs they do not accept str. Note that even a 'wrong' object with a buffer protocol that is not one of these three exact types is rejected.

Source

Thrown at Lib/asyncio/selector_events.py:1063

            keep_open = self._protocol.eof_received()
        except (SystemExit, KeyboardInterrupt):
            raise
        except BaseException as exc:
            self._fatal_error(
                exc, 'Fatal error: protocol.eof_received() call failed.')
            return

        if keep_open:
            # We're keeping the connection open so the
            # protocol can write more, but we still can't
            # receive more, so remove the reader callback.
            self._loop._remove_reader(self._sock_fd)
        else:
            self.close()

    def write(self, data):
        if not isinstance(data, (bytes, bytearray, memoryview)):
            raise TypeError(f'data argument must be a bytes, bytearray, or memoryview '
                            f'object, not {type(data).__name__!r}')
        if self._eof:
            raise RuntimeError('Cannot call write() after write_eof()')
        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

        if not self._buffer:
            # Optimization: try to send now.
            try:
                n = self._sock.send(data)

View on GitHub (pinned to bc6749cc3b)

Solutions

Encode strings before writing: transport.write(text.encode('utf-8')).
Convert arbitrary objects to bytes explicitly (bytes(obj), obj.tobytes(), json.dumps(...).encode()).
Add a small wrapper or type guard in your protocol layer that asserts isinstance(data, (bytes, bytearray, memoryview)) before delegating to the transport.

Example fix

// before
writer.write(f"ping\n")  # str -> TypeError

// after
writer.write(f"ping\n".encode("utf-8"))
await writer.drain()

Defensive patterns

Strategy: type-guard

Validate before calling

from typing import Any

def to_bytes(data: Any) -> bytes:
    if isinstance(data, str):
        return data.encode('utf-8')
    if isinstance(data, (bytes, bytearray, memoryview)):
        return bytes(data)
    raise TypeError(f'cannot serialize {type(data).__name__} for transport')

Type guard

BytesLike = (bytes, bytearray, memoryview)

def is_writable_data(data) -> bool:
    return isinstance(data, BytesLike)

Try / catch

try:
    transport.write(data)
except TypeError as e:
    if 'data argument' in str(e):
        transport.write(str(data).encode('utf-8'))
    else:
        raise

Prevention

Encode at the edge: convert str to bytes at the API boundary, never inside transport code.
Use type annotations (bytes) plus mypy to catch str/bytes confusion statically.
Write a send() wrapper in your protocol class that type-checks payloads once.

When it happens

Trigger: Calling transport.write('hello') (str), transport.write(123), transport.write(['a','b']), or an arbitrary object, on a transport returned by loop.create_connection()/create_server() (typically wrapped by StreamWriter.write which forwards to the transport).

Common situations: Forgetting .encode() on a string when migrating from a library whose send() accepted str (e.g. websocket or zeromq wrappers); passing serialized-but-not-yet-encoded objects; passing a numpy array or other buffer type instead of its bytes() representation.

Related errors

AI-assisted analysis of python/cpython@bc6749cc3b (2026-08-14). Data as JSON: /api/errors/2e3b10e350eadbe3. Report an issue: GitHub.