A :class:`logging.Logger` logger for the app.

        This can be used to log messages in a format as defined in the
        app configuration, for example,

        .. code-block:: python

            app.logger.debug("Request method %s", request.method)
            app.logger.error("Error, of some kind")

def logger(self) -> Logger:
        """A :class:`logging.Logger` logger for the app.

        This can be used to log messages in a format as defined in the
        app configuration, for example,

        .. code-block:: python

            app.logger.debug("Request method %s", request.method)
            app.logger.error("Error, of some kind")

        """
        if self._logger is None:
            self._logger = create_logger(self)
        return self._logger