How do you create custom exception handlers in FastAPI?

Use @app.exception_handler(ExceptionClass) to catch specific exception types globally and return custom JSON responses. This is cleaner than wrapping every route in try/except.

from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse
from fastapi.exceptions import RequestValidationError
from starlette.exceptions import HTTPException as StarletteHTTPException

app = FastAPI()

# Custom exception class
class ItemNotFoundError(Exception):
    def __init__(self, item_id: int):
        self.item_id = item_id

# Handler for our custom exception
@app.exception_handler(ItemNotFoundError)
async def item_not_found_handler(request: Request, exc: ItemNotFoundError):
    return JSONResponse(
        status_code=404,
        content={"error": "not_found", "item_id": exc.item_id},
    )

# Override the default validation error handler
@app.exception_handler(RequestValidationError)
async def validation_exception_handler(request: Request, exc: RequestValidationError):
    return JSONResponse(
        status_code=422,
        content={
            "error": "validation_failed",
            "detail": exc.errors(),
            "body": exc.body,
        },
    )

# Override generic HTTP exception handler
@app.exception_handler(StarletteHTTPException)
async def http_exception_handler(request: Request, exc: StarletteHTTPException):
    return JSONResponse(
        status_code=exc.status_code,
        content={"error": "http_error", "detail": exc.detail},
    )

@app.get("/items/{item_id}")
def get_item(item_id: int):
    if item_id == 999:
        raise ItemNotFoundError(item_id=item_id)  # triggers custom handler
    return {"item_id": item_id}

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