How do I write Flask’s excellent debug log message to a file in production?

I don’t know why it’s not working but I can tell how am doing this.

First of all, you don’t need to set the level of app.logger. So remove this line app.logger.setLevel().

You want to save exception and return error page for every view. It is a lot of work to write this code everywhere. Flask provides a method to do this. Define an errorhandler method like this.

    @app.errorhandler(500)
    def internal_error(exception):
        app.logger.error(exception)
        return render_template('500.html'), 500

Whenever a view raises an exception, this method will be called and passed the exception as argument. Python logging provides exception method that is used to save full traceback of the exception.

Since this handles all exception, you don’t even need to put code in try/except block. Though, if you want to do something before calling the errorhandler(for e.g. rollback session or transaction) then do this:

    try:
        #code
    except:
        #code
        raise

If you would like the date and time added for each entry in your log file, the following code can be used (in place of the similar code featured in the question).

if app.debug is not True:   
    import logging
    from logging.handlers import RotatingFileHandler
    file_handler = RotatingFileHandler('python.log', maxBytes=1024 * 1024 * 100, backupCount=20)
    file_handler.setLevel(logging.ERROR)
    formatter = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s")
    file_handler.setFormatter(formatter)
    app.logger.addHandler(file_handler)