How to handle extra data error in FastAPI

Question

So I have the following FastAPI code:

from fastapi import FastAPI

app = FastAPI

class Demo(BaseModel):
    content: str = None
    
@app.post("/demo")
async def demoFunc(d:Demo):
    return d.content

Issue is that when I send a request to this API with extra data like:

data = {"content":"some text here"}aaaa

OR

data = {"content":"some text here"aaaaaa}

resp = requests.post(url, json=data)

It throws an error with status code 422 unprocessable entity error with Actual("some text here") and Extra("aaaaa") data in the return field in case of data = {"content":"some text here"}aaaa:

{
  "detail": [
    {
      "loc": [
        "body",
        47
      ],
      "msg": "Extra data: line 4 column 2 (char 47)",
      "type": "value_error.jsondecode",
      "ctx": {
        "msg": "Extra data",
        "doc": "{\n  \"content\": \"some text here\"}aaaaa",
        "pos": 47,
        "lineno": 4,
        "colno": 2
      }
    }
  ]
}

I tried to put the line app=FastAPI in a try-catch block, however, it doesn't work. Is there any way I can handle this issue with own response instead of the above mentioned auto response? Something like this:

{"error": {"message": "Invalid JSON body"},
                         "status": 0}

Answer 1

What exactly is that you are trying to achieve? You are passing an invalid JSON and the server correctly responds with the 422 unprocessable entity error. Your test client shouldn't be able to run at all, without throwing an invalid syntax error. So, I'm guessing you tried that through OpenAPI and received the relevant 422 error.

If what you actually want is to handle the error, in order to customise the error or something, you can override the request validation exception, as described in the documentation (also, have a look at this discussion). Below is an example.

from fastapi import FastAPI, Body, Request, status
from fastapi.encoders import jsonable_encoder
from fastapi.exceptions import RequestValidationError
from fastapi.responses import JSONResponse
from pydantic import BaseModel

app = FastAPI()

@app.exception_handler(RequestValidationError)
async def validation_exception_handler(request: Request, exc: RequestValidationError):
    return JSONResponse(
        status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
        content=jsonable_encoder({"detail": exc.errors(),
                "body": exc.body,
                 "custom msg": {"Your error message"}}),
    )

class Demo(BaseModel):
    content: str = None

@app.post("/demo")
async def demoFunc(d:Demo):
    return d.content

Or, you could also return a PlainTextResponse with any custom message:

from fastapi.responses import PlainTextResponse
@app.exception_handler(RequestValidationError)
async def validation_exception_handler(request, exc):
    return PlainTextResponse(str(exc), status_code=400)