It seems to me that is somewhat of a design flaw.

Say I have a number of threads, each serving a client connection. Now when
the thread starts, I can say:

      logbook.Processor(self.extra_info).push_thread()

This processor would add the user id to the record. Now, in various places
within the thread, I can simply call the logbook log functions, with the
user id always being automatically injected.

The problem is getting the extra info formatted. The handler that I setup
at the application-level knows nothing of these specific extra fields of a
sub-component of the app, nor should it.

I am considering attaching a handler to the logger that only reformats the
message:

    class ExtraFormattingHandler(logbook.Handler):
        def __init__(self):
            super(ExtraFormattingHandler, self).__init__(bubble=True)

        def emit(self, record):            
             record.message = '....'

    log = logbook.Logger(__name__)
    log.handlers.append(ExtraFormattingHandler())

However, that feels a bit hacky. Is there a better way to deal with this?