aio-libs/aiohttp · error · TypeError

Unable to decode.

Error message

Unable to decode.

What it means

Raised unconditionally by BodyPartReaderPayload.decode(). BodyPartReaderPayload is a streaming Payload wrapper around a BodyPartReader; it cannot materialize its full content as a string because the underlying stream may be arbitrarily large and can only be consumed once. The method exists only to satisfy the Payload ABC contract and deliberately rejects synchronous decoding.

Source

Thrown at aiohttp/multipart.py:652

@payload_type(BodyPartReader, order=Order.try_first)
class BodyPartReaderPayload(Payload):
    _value: BodyPartReader
    # _autoclose = False (inherited) - Streaming reader that may have resources

    def __init__(self, value: BodyPartReader, *args: Any, **kwargs: Any) -> None:
        super().__init__(value, *args, **kwargs)

        params: dict[str, str] = {}
        if value.name is not None:
            params["name"] = value.name
        if value.filename is not None:
            params["filename"] = value.filename

        if params:
            self.set_content_disposition("attachment", True, **params)

    def decode(self, encoding: str = "utf-8", errors: str = "strict") -> str:
        raise TypeError("Unable to decode.")

    async def as_bytes(self, encoding: str = "utf-8", errors: str = "strict") -> bytes:
        """Raises TypeError as body parts should be consumed via write().

        This is intentional: BodyPartReader payloads are designed for streaming
        large data (potentially gigabytes) and must be consumed only once via
        the write() method to avoid memory exhaustion. They cannot be buffered
        in memory for reuse.
        """
        raise TypeError("Unable to read body part as bytes. Use write() to consume.")

    async def write(self, writer: AbstractStreamWriter) -> None:
        field = self._value
        while chunk := await field.read_chunk(size=DEFAULT_CHUNK_SIZE):
            async for d in field.decode_iter(chunk):
                await writer.write(d)

View on GitHub (pinned to c0ef574e29)

Solutions

1. Do not call decode() on streaming payloads — consume them via `await payload.write(writer)` instead.
2. If you need bytes, switch to a non-streaming payload (BytesPayload) by first reading the part into memory: `data = await part.read(decode=True)`.
3. Type-check with isinstance(payload, BodyPartReaderPayload) before calling decode() in generic code.

Example fix

// before
text = payload.decode()
// after
buf = io.BytesIO()
await payload.write(StreamWriter(buf))  # stream-consume instead
text = buf.getvalue().decode('utf-8')

from aiohttp.multipart import BodyPartReaderPayload
if isinstance(payload, BodyPartReaderPayload):
    raise TypeError('use write() to consume streaming payloads')

from aiohttp.multipart import BodyPartReaderPayload

def is_streaming_payload(p) -> bool:
    return isinstance(p, BodyPartReaderPayload)

try:
    text = payload.decode()
except TypeError:
    # streaming payload — consume via write()
    buf = io.BytesIO()
    await payload.write(StreamWriter(buf))
    text = buf.getvalue().decode('utf-8')

When it happens

Trigger: Calling `.decode()` on a payload obtained from a BodyPartReader (e.g. when a BodyPartReader is registered as a payload via @payload_type and then someone calls payload.decode()). Happens in generic payload-handling code that assumes all Payload subclasses support decode().

Common situations: Generic middleware that calls `.decode()` on every payload; test helpers serializing payloads to strings; copying a BodyPartReader into another request body via a path that tries to buffer the whole thing.

Related errors

• Unable to read body part as bytes. Use write() to consume.
• Only io.IOBase, multidict and (name, file) pairs allowed, us
• Can not serialize value type: %r headers: %r value: %r
• Could not find starting boundary {self._boundary!r}
• Cannot create payload from %r

AI-assisted analysis of aio-libs/aiohttp@c0ef574e29 (2026-08-04). Data as JSON: /data/errors/9a1938c9700e73c1.json. Report an issue: GitHub.