Validation error responses in FastAPI

Introduction

Validation error responses tell users what went wrong when they send wrong data. This helps fix mistakes quickly.

When a user sends a form with missing or wrong information.

When an API receives data that does not match the expected format.

When you want to guide users to correct their input before processing.

When building a web service that needs to be clear about input rules.

Syntax

FastAPI

from fastapi import FastAPI, Request
from pydantic import BaseModel
from fastapi.responses import JSONResponse
from fastapi.exceptions import RequestValidationError

app = FastAPI()

@app.exception_handler(RequestValidationError)
async def validation_exception_handler(request: Request, exc: RequestValidationError):
    return JSONResponse(
        status_code=422,
        content={"detail": exc.errors(), "body": exc.body},
    )

This code shows how to catch validation errors and send a clear JSON response.

FastAPI uses Pydantic models to check data automatically.

Examples

This example defines a data model and an endpoint that expects valid data. If data is wrong, FastAPI sends a validation error automatically.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class Item(BaseModel):
    name: str
    price: float

@app.post("/items/")
async def create_item(item: Item):
    return item

This example shows how to customize the error response message and status code.

FastAPI

from fastapi import FastAPI, Request
from fastapi.exceptions import RequestValidationError
from fastapi.responses import JSONResponse

app = FastAPI()

@app.exception_handler(RequestValidationError)
async def custom_validation_handler(request: Request, exc: RequestValidationError):
    return JSONResponse(
        status_code=400,
        content={"message": "Oops! Your data is not valid.", "errors": exc.errors()},
    )

Sample Program

This FastAPI app defines a User model and an endpoint to create users. If the input data is invalid, it returns a JSON response with details about the errors.

FastAPI

from fastapi import FastAPI, Request
from pydantic import BaseModel
from fastapi.exceptions import RequestValidationError
from fastapi.responses import JSONResponse

app = FastAPI()

class User(BaseModel):
    username: str
    age: int

@app.post("/users/")
async def create_user(user: User):
    return {"message": "User created", "user": user}

@app.exception_handler(RequestValidationError)
async def validation_exception_handler(request: Request, exc: RequestValidationError):
    return JSONResponse(
        status_code=422,
        content={"detail": exc.errors(), "body": exc.body},
    )

Important Notes

FastAPI automatically validates request data using Pydantic models.

You can customize error responses by writing an exception handler for RequestValidationError.

Validation errors usually return HTTP status 422 (Unprocessable Entity).

Summary

Validation error responses help users fix wrong input.

FastAPI uses Pydantic to check data and sends errors automatically.

You can customize error messages by handling RequestValidationError.