Custom serialization for bytes - something instead of a string?

[treykeown]

I think it would be cool to serialize bytes on the Python side, and automatically have that show up as a Uint8Array on the JavaScript side. It should be possible in JS with a custom reviver, but can't I figure out how to control serialization of bytes. I'm trying to get something like this:

b"\xCA\xFE" -> {"__type__": "bytes", "value": "yv4="}

Is it possible to control how bytes are serialized? Seems like the default msgspec behavior is used.

[vinicq]

Override the bytes encoder via type_encoders on the Litestar app.

import base64
from litestar import Litestar

def encode_bytes(value: bytes) -> dict[str, str]:
    return {"__type__": "bytes", "value": base64.b64encode(value).decode("ascii")}

app = Litestar(
    route_handlers=[...],
    type_encoders={bytes: encode_bytes},
)

That gives you the wrapped shape exactly. msgspec calls the encoder when it hits a bytes value inside any returned structure.

If you want to read that shape back into bytes when it comes in as a request body, you'll need a matching decoder (custom Pydantic/msgspec model with a field validator, or a type_decoders entry), since the encoder only covers the outbound side.

One gotcha. If a handler returns raw bytes as the response body itself (not nested inside another structure), Litestar treats it as binary content and short-circuits the encoder. The encoder fires when bytes appears inside a returned dict, list, or model, which is exactly what you want for the JS Uint8Array reviver path.