Proof of concept using a logging handler and LogRecord "extras" attribute to manage a progress bar.

gistfile1.py

# Copyright (c) 2015 David Warde-Farley.
# 
# Permission is granted to use this code under the MIT license: 
# http://opensource.org/licenses/mit-license.php

import logging
import progressbar
import time


class ProgressBarHandler(logging.Handler):
    def __init__(self, max_val_attr, curr_val_attr, widgets=None,
                 start_predicate=None, update_predicate=None,
                 level=logging.DEBUG):
        super(ProgressBarHandler, self).__init__(level)
        self.max_val_attr = max_val_attr
        self.curr_val_attr = curr_val_attr
        self.widgets = widgets
        self.start_predicate = (start_predicate if start_predicate is not None
                                else lambda x: hasattr(x, self.max_val_attr))
        self.update_predicate = (update_predicate
                                 if update_predicate is not None
                                 else lambda x: hasattr(x, self.curr_val_attr))
        self.level = level

    def handle(self, record):
        if self.start_predicate(record):
            maxval = getattr(record, self.max_val_attr)
            self.progress_bar = progressbar.ProgressBar(
                maxval=maxval,
                widgets=self.widgets).start()
        elif self.update_predicate(record):
            currval = getattr(record, self.curr_val_attr)
            self.progress_bar.update(currval)


if __name__ == "__main__":
    log = logging.getLogger(__name__)
    log.setLevel(logging.DEBUG)
    log.addHandler(ProgressBarHandler(max_val_attr='num_things',
                                      curr_val_attr='done_so_far'))
    log.debug('A setup message', extra={'num_things': 10000})
    for i in range(10000):
        log.info('Irrelevant stuff')
        log.debug('More irrelevant stuff')
        log.debug("Look ma, no progressbar calls!", extra={'done_so_far': i})
        time.sleep(0.2)