Getting request context for error logging in Flask?

Question

The Flask tutorial has an example of emailing yourself when an error occurs. I would like to add some information from request, but kept receiving an error:

RuntimeError: working outside of request context

Here is what I have:

if  __name__ == '__main__':
    if not app.debug:  

        # create mail handler
        import logging
        from logging.handlers import SMTPHandler
        mail_handler = SMTPHandler('127.0.0.1',
                               'server-error@example.com',
                               ['admin@example.com'], 'YourApplication Failed')

        # Log format
        from logging import Formatter
        mail_handler.setFormatter(Formatter('''
        Message type:       %(levelname)s
        Location:           %(pathname)s:%(lineno)d
        Module:             %(module)s
        Function:           %(funcName)s
        Time:               %(asctime)s

        Message:

        %(message)s
        ''' % request.headers ))   # Added request here

        # Attach log handler to app
        mail_handler.setLevel(logging.ERROR)
        app.logger.addHandler(mail_handler)

How do I get the request context for logging?

Answer 1

You actually need to add a filter to add what you want to the logger:

import logging

class ContextualFilter(logging.Filter):
    def filter(self, log_record):
        log_record.url = request.path
        log_record.method = request.method
        log_record.ip = request.environ.get("REMOTE_ADDR")
        log_record.headers = request.headers

        return True

Then you can register the filter with the app's logger:

context_provider = ContextualFilter()
app.logger.addFilter(context_provider)

And use the extra keys you added to the context in your formatter:

mail_handler.setFormatter(Formatter('''
    Message type:       %(levelname)s
    Location:           %(pathname)s:%(lineno)d
    Module:             %(module)s
    Function:           %(funcName)s
    Time:               %(asctime)s
    URL:                %(url)s
    Method:             %(method)s
    Headers:            %(headers)s

    Message:

    %(message)s
    '''))

Why can't I just add request.headers to my formatter

Two reasons:

1. There is no request context when you're setting up the logger since no request is inbound
2. Even if there was, that code won't actually do what you want it to do. It will add the request headers from the request in scope when you set up the logger (so all requests will be that first request).

See also: https://realpython.com/blog/python/python-web-applications-with-flask-part-iii/