starlette/docs/debug.md

You can use Starlette's DebugMiddleware to display simple error tracebacks in the browser.

from starlette.debug import DebugMiddleware


class App:
    def __init__(self, scope):
        self.scope = scope

    async def __call__(self, receive, send):
        raise RuntimeError('Something went wrong')


app = DebugMiddleware(App)

For a more complete handling of exception cases you may wish to use Starlette's ExceptionMiddleware class instead, which also includes optional debug handling.