Logging
Python already has an excellent built-in logging module. You may want to use the logging template here.
The logging module lets you specify a level of importance; during debugging you can log everything, while during normal operation you might only log critical things. You can switch things off and on.
Most people just use basic print statements to debug, and then remove the print statements. It's better to leave them in, but disable them; then, when you have another bug, you can just re-enable everything and look your logs over.
This can be the best possible way to debug programs that need to do things quickly, such as networking programs that need to respond before the other end of the network connection times out and goes away. You might not have much time to single-step a debugger; but you can just let your code run, and log everything, then pore over the logs and figure out what's really happening.
EDIT: The original URL for the templates was: http://aymanh.com/python-debugging-techniques
This page is missing so I replaced it with a reference to the snapshot saved at archive.org: http://web.archive.org/web/20120819135307/http://aymanh.com/python-debugging-techniques
In case it disappears again, here are the templates I mentioned. This is code taken from the blog; I didn't write it.
import logging
import optparse
LOGGING_LEVELS = {'critical': logging.CRITICAL,
'error': logging.ERROR,
'warning': logging.WARNING,
'info': logging.INFO,
'debug': logging.DEBUG}
def main():
parser = optparse.OptionParser()
parser.add_option('-l', '--logging-level', help='Logging level')
parser.add_option('-f', '--logging-file', help='Logging file name')
(options, args) = parser.parse_args()
logging_level = LOGGING_LEVELS.get(options.logging_level, logging.NOTSET)
logging.basicConfig(level=logging_level, filename=options.logging_file,
format='%(asctime)s %(levelname)s: %(message)s',
datefmt='%Y-%m-%d %H:%M:%S')
# Your program goes here.
# You can access command-line arguments using the args variable.
if __name__ == '__main__':
main()
And here is his explanation of how to use the above. Again, I don't get the credit for this:
By default, the logging module prints critical, error and warning messages. To change this so that all levels are printed, use:
$ ./your-program.py --logging=debug
To send log messages to a file called debug.log, use:
$ ./your-program.py --logging-level=debug --logging-file=debug.log
