fastapi/docs/en/docs/advanced/json-base64-bytes.md

JSON with Bytes as Base64

If your app needs to receive and send JSON data, but you need to include binary data in it, you can encode it as base64.

Base64 vs Files

Consider first if you can use Request Files{.internal-link target=_blank} for uploading binary data and Custom Response - FileResponse{.internal-link target=_blank} for sending binary data, instead of encoding it in JSON.

JSON can only contain UTF-8 encoded strings, so it can't contain raw bytes.

Base64 can encode binary data in strings, but to do it, it needs to use more characters than the original binary data, so it would normally be less efficient than regular files.

Use base64 only if you definitely need to include binary data in JSON, and you can't use files for that.

Pydantic bytes

You can declare a Pydantic model with bytes fields, and then use val_json_bytes in the model config to tell it to use base64 to validate input JSON data, as part of that validation it will decode the base64 string into bytes.

{* ../../docs_src/json_base64_bytes/tutorial001_py310.py ln[1:9,29:35] hl[9] *}

If you check the /docs, they will show that the field data expects base64 encoded bytes:

You could send a request like:

{
    "description": "Some data",
    "data": "aGVsbG8="
}

/// tip

aGVsbG8= is the base64 encoding of hello.

///

And then Pydantic will decode the base64 string and give you the original bytes in the data field of the model.

You will receive a response like:

{
  "description": "Some data",
  "content": "hello"
}

Pydantic bytes for Output Data

You can also use bytes fields with ser_json_bytes in the model config for output data, and Pydantic will serialize the bytes as base64 when generating the JSON response.

{* ../../docs_src/json_base64_bytes/tutorial001_py310.py ln[1:2,12:16,29,38:41] hl[16] *}

Pydantic bytes for Input and Output Data

And of course, you can use the same model configured to use base64 to handle both input (validate) with val_json_bytes and output (serialize) with ser_json_bytes when receiving and sending JSON data.

{* ../../docs_src/json_base64_bytes/tutorial001_py310.py ln[1:2,19:26,29,44:46] hl[23:26] *}