def errorhandler(
        self, code_or_exception: type[Exception] | int
    ) -> t.Callable[[T_error_handler], T_error_handler]:
        """Register a function to handle errors by code or exception class.

        A decorator that is used to register a function given an
        error code.  Example::

            @app.errorhandler(404)
            def page_not_found(error):
                return 'This page does not exist', 404

        You can also register handlers for arbitrary exceptions::

            @app.errorhandler(DatabaseError)
            def special_exception_handler(error):
                return 'Database connection failed', 500

        This is available on both app and blueprint objects. When used on an app, this
        can handle errors from every request. When used on a blueprint, this can handle
        errors from requests that the blueprint handles. To register with a blueprint
        and affect every request, use :meth:`.Blueprint.app_errorhandler`.

        .. versionadded:: 0.7
            Use :meth:`register_error_handler` instead of modifying
            :attr:`error_handler_spec` directly, for application wide error
            handlers.

        .. versionadded:: 0.7
           One can now additionally also register custom exception types
           that do not necessarily have to be a subclass of the
           :class:`~werkzeug.exceptions.HTTPException` class.

        :param code_or_exception: the code as integer for the handler, or
                                  an arbitrary exception
        """

        def decorator(f: T_error_handler) -> T_error_handler:
            self.register_error_handler(code_or_exception, f)
            return f

        return decorator
Register a function to handle errors by code or exception class.

        A decorator that is used to register a function given an
        error code.  Example::

            @app.errorhandler(404)
            def page_not_found(error):
                return 'This page does not exist', 404

        You can also register handlers for arbitrary exceptions::

            @app.errorhandler(DatabaseError)
            def special_exception_handler(error):
                return 'Database connection failed', 500

        This is available on both app and blueprint objects. When used on an app, this
        can handle errors from every request. When used on a blueprint, this can handle
        errors from requests that the blueprint handles. To register with a blueprint
        and affect every request, use :meth:`.Blueprint.app_errorhandler`.

        .. versionadded:: 0.7
            Use :meth:`register_error_handler` instead of modifying
            :attr:`error_handler_spec` directly, for application wide error
            handlers.

        .. versionadded:: 0.7
           One can now additionally also register custom exception types
           that do not necessarily have to be a subclass of the
           :class:`~werkzeug.exceptions.HTTPException` class.

        :param code_or_exception: the code as integer for the handler, or
                                  an arbitrary exception
        
src/flask/sansio/scaffold.py