_exception_handler.py

from __future__ import annotations

import typing

from starlette._utils import is_async_callable
from starlette.concurrency import run_in_threadpool
from starlette.exceptions import HTTPException
from starlette.requests import Request
from starlette.types import (
    ASGIApp,
    ExceptionHandler,
    HTTPExceptionHandler,
    Message,
    Receive,
    Scope,
    Send,
    WebSocketExceptionHandler,
)
from starlette.websockets import WebSocket

ExceptionHandlers = typing.Dict[typing.Any, ExceptionHandler]
StatusHandlers = typing.Dict[int, ExceptionHandler]


def _lookup_exception_handler(
    exc_handlers: ExceptionHandlers, exc: Exception
) -> ExceptionHandler | None:
    for cls in type(exc).__mro__:
        if cls in exc_handlers:
            return exc_handlers[cls]
    return None


def wrap_app_handling_exceptions(app: ASGIApp, conn: Request | WebSocket) -> ASGIApp:
    exception_handlers: ExceptionHandlers
    status_handlers: StatusHandlers
    try:
        exception_handlers, status_handlers = conn.scope["starlette.exception_handlers"]
    except KeyError:
        exception_handlers, status_handlers = {}, {}

    async def wrapped_app(scope: Scope, receive: Receive, send: Send) -> None:
        response_started = False

        async def sender(message: Message) -> None:
            nonlocal response_started

            if message["type"] == "http.response.start":
                response_started = True
            await send(message)

        try:
            await app(scope, receive, sender)
        except Exception as exc:
            handler = None

            if isinstance(exc, HTTPException):
                handler = status_handlers.get(exc.status_code)

            if handler is None:
                handler = _lookup_exception_handler(exception_handlers, exc)

            if handler is None:
                raise exc

            if response_started:
                msg = "Caught handled exception, but response already started."
                raise RuntimeError(msg) from exc

            if scope["type"] == "http":
                nonlocal conn
                handler = typing.cast(HTTPExceptionHandler, handler)
                conn = typing.cast(Request, conn)
                if is_async_callable(handler):
                    response = await handler(conn, exc)
                else:
                    response = await run_in_threadpool(handler, conn, exc)
                await response(scope, receive, sender)
            elif scope["type"] == "websocket":
                handler = typing.cast(WebSocketExceptionHandler, handler)
                conn = typing.cast(WebSocket, conn)
                if is_async_callable(handler):
                    await handler(conn, exc)
                else:
                    await run_in_threadpool(handler, conn, exc)

    return wrapped_app