def preprocess_request(self) -> ft.ResponseReturnValue | None:
        """Called before the request is dispatched. Calls
        :attr:`url_value_preprocessors` registered with the app and the
        current blueprint (if any). Then calls :attr:`before_request_funcs`
        registered with the app and the blueprint.

        If any :meth:`before_request` handler returns a non-None value, the
        value is handled as if it was the return value from the view, and
        further request handling is stopped.
        """
        names = (None, *reversed(request.blueprints))

        for name in names:
            if name in self.url_value_preprocessors:
                for url_func in self.url_value_preprocessors[name]:
                    url_func(request.endpoint, request.view_args)

        for name in names:
            if name in self.before_request_funcs:
                for before_func in self.before_request_funcs[name]:
                    rv = self.ensure_sync(before_func)()

                    if rv is not None:
                        return rv  # type: ignore[no-any-return]

        return None
Called before the request is dispatched. Calls
        :attr:`url_value_preprocessors` registered with the app and the
        current blueprint (if any). Then calls :attr:`before_request_funcs`
        registered with the app and the blueprint.

        If any :meth:`before_request` handler returns a non-None value, the
        value is handled as if it was the return value from the view, and
        further request handling is stopped.
        
src/flask/app.py