How to customise error response in FastAPI?

by

You are passing an invalid JSON, and hence, 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 posted the request through the interactive autodocs provided by Swagger UI at /docs, 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 handler, as described in the documentation (have a look at this discussion, as well as this answer and this answer that demonstrates how to customise the RequestValidationError for specific routes only).

Working 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(),  # optionally include the errors
                "body": exc.body,
                 "custom msg": {"Your error message"}}),
    )

class Demo(BaseModel):
    content: str = None

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

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

from fastapi.responses import PlainTextResponse

@app.exception_handler(RequestValidationError)
async def validation_exception_handler(request, exc):
    return PlainTextResponse(str(exc), status_code=422)

More Related Contents:

  1. How to add both file and JSON body in a FastAPI POST request?
  2. How to return data in JSON format using FastAPI?
  3. FastAPI is very slow in returning a large amount of JSON data
  4. How to create a FastAPI endpoint that can accept either Form or JSON body?
  5. How to return a custom Response when a Header is absent from the Request using FastAPI?
  6. How to download a large file using FastAPI?
  7. How to exclude Optional unset values from a Pydantic model using FastAPI?
  8. How to use a Pydantic model with Form data in FastAPI?
  9. FastAPI – GET Request with Pydantic List field
  10. How to customise error response for a specific route in FastAPI?
  11. How do I write JSON data to a file?
  12. Single vs double quotes in JSON
  13. Reading rather large JSON files [duplicate]
  14. Python/Json:Expecting property name enclosed in double quotes
  15. How to get JSON from webpage into Python script
  16. Python’s json module, converts int dictionary keys to strings
  17. Django REST Framework POST nested objects
  18. Serializing a Python namedtuple to json
  19. flattening nested Json in pandas data frame
  20. Python: Trying to Deserialize Multiple JSON objects in a file with each object spanning multiple but consistently spaced number of lines
  21. How to convert to a Python datetime object with JSON.loads?
  22. Cast Flask form value to int
  23. Serialising an Enum member to JSON
  24. Can json.loads ignore trailing commas?
  25. How to return a PDF file from in-memory buffer using FastAPI?
  26. Convert JSON to SQLite in Python – How to map json keys to database columns properly?
  27. How to define multiple API endpoints in FastAPI with different paths but the same path parameter?
  28. How to make a large file accessible to external APIs?
  29. How to convert csv to json in python?
  30. How to return and download Excel file using FastAPI?

Leave a Comment