aio-libs/aiohttp · error · TypeError

data argument must be byte-ish (%r)

Error message

data argument must be byte-ish (%r)

What it means

Raised as TypeError in ClientWebSocketResponse.send_bytes() when data is not a bytes-like object. Binary frames require raw bytes; the accepted types are bytes, bytearray, and memoryview.

Source

Thrown at aiohttp/client_ws.py:294

    async def pong(self, message: bytes = b"") -> None:
        await self._writer.send_frame(message, WSMsgType.PONG)

    async def send_frame(
        self, message: bytes, opcode: WSMsgType, compress: int | None = None
    ) -> None:
        """Send a frame over the websocket."""
        await self._writer.send_frame(message, opcode, compress)

    async def send_str(self, data: str, compress: int | None = None) -> None:
        if not isinstance(data, str):
            raise TypeError("data argument must be str (%r)" % type(data))
        await self._writer.send_frame(
            data.encode("utf-8"), WSMsgType.TEXT, compress=compress
        )

    async def send_bytes(self, data: bytes, compress: int | None = None) -> None:
        if not isinstance(data, (bytes, bytearray, memoryview)):
            raise TypeError("data argument must be byte-ish (%r)" % type(data))
        await self._writer.send_frame(data, WSMsgType.BINARY, compress=compress)

    async def send_json(
        self,
        data: Any,
        compress: int | None = None,
        *,
        dumps: JSONEncoder = DEFAULT_JSON_ENCODER,
    ) -> None:
        await self.send_str(dumps(data), compress=compress)

    async def send_json_bytes(
        self,
        data: Any,
        compress: int | None = None,
        *,
        dumps: JSONBytesEncoder,
    ) -> None:

View on GitHub (pinned to c0ef574e29)

Solutions

1. Pass bytes/bytearray/memoryview to send_bytes.
2. Encode str first: data.encode('utf-8') or just use send_str.
3. Convert buffer protocol objects: bytes(arr) or memoryview(arr).
4. For JSON use send_json_bytes with an orjson-style encoder.

Example fix

# before
await ws.send_bytes('hello')      # str
await ws.send_bytes([1,2,3])      # list
# after
await ws.send_bytes(b'hello')
await ws.send_bytes(bytes([1,2,3]))
# or for text
await ws.send_str('hello')

if not isinstance(data, (bytes, bytearray, memoryview)):
    raise TypeError(f'send_bytes requires bytes-like, got {type(data).__name__}')
await ws.send_bytes(data)

def is_ws_binary(data) -> bool:
    return isinstance(data, (bytes, bytearray, memoryview))

When it happens

Trigger: Fires at line 293-294 when isinstance(data, (bytes, bytearray, memoryview)) is False. Triggered by passing a str ('hello'), an int, a list of ints, or None to send_bytes.

Common situations: Passing a str to send_bytes (use send_str instead); passing JSON as a string instead of bytes; passing a numpy array or other buffer-like object without conversion.

Related errors

• data argument must be str (%r)
• Compress wbits must between 9 and 15, zlib does not support
• Concurrent call to receive() is not allowed
• filename must be an instance of str. Got: %s
• content_type must be an instance of str. Got: %s

AI-assisted analysis of aio-libs/aiohttp@c0ef574e29 (2026-08-04). Data as JSON: /data/errors/ab4236ef6f4a84fb.json. Report an issue: GitHub.