tiangolo.fastapi/fastapi/exception_handlers.py

from fastapi.encoders import jsonable_encoder
from fastapi.exceptions import RequestValidationError, WebSocketRequestValidationError
from fastapi.utils import is_body_allowed_for_status_code
from fastapi.websockets import WebSocket
from starlette.exceptions import HTTPException
from starlette.requests import Request
from starlette.responses import JSONResponse, Response
from starlette.status import WS_1008_POLICY_VIOLATION


async def http_exception_handler(request: Request, exc: HTTPException) -> Response:
    """Default handler for `HTTPException`.

    Returns a plain `Response` (no body) when the status code disallows a
    response body (e.g. 204, 304), otherwise returns a JSON response with
    the exception's `detail` field.
    """
    headers = getattr(exc, "headers", None)
    if not is_body_allowed_for_status_code(exc.status_code):
        return Response(status_code=exc.status_code, headers=headers)
    return JSONResponse(
        {"detail": exc.detail}, status_code=exc.status_code, headers=headers
    )


async def request_validation_exception_handler(
    request: Request, exc: RequestValidationError
) -> JSONResponse:
    """Default handler for `RequestValidationError`.

    Returns a 422 JSON response whose body contains the list of validation
    errors produced by Pydantic.
    """
    return JSONResponse(
        status_code=422,
        content={"detail": jsonable_encoder(exc.errors())},
    )


async def websocket_request_validation_exception_handler(
    websocket: WebSocket, exc: WebSocketRequestValidationError
) -> None:
    """Default handler for `WebSocketRequestValidationError`.

    Closes the WebSocket connection with policy-violation code 1008 and
    includes the validation errors in the close reason.
    """
    await websocket.close(
        code=WS_1008_POLICY_VIOLATION, reason=jsonable_encoder(exc.errors())
    )