Beginning Python - From Novice to Professional

386 CHAPTER 19 ■ PLAYFUL PROGRAMMING
the data accumulate, for that matter). A very simple form of logging can be done with the print
statement. Just put a statement like this at the beginning of your program:
log = open('logfile.txt', 'w')
You can then later put any interesting information about the state of your program into
this file as follows:
print >> log, ('Downloading file from URL %s' % url)
text = urllib.urlopen(url).read()
print >> log, 'File successfully downloaded'
This approach won’t work well if your program crashes during the download. It would be
safer if you opened and closed your file for every log statement (or, at least, flushed the file after
writing). Then, if your program crashed, you could see that the last line in your log file said
“Downloading file from . . .” and you would know that the download wasn’t successful.
The way to go, actually, is using the logging module in the standard library. Basic usage is
pretty straightforward, as demonstrated by the program in Listing 19-3.
Listing 19-3. A Program Using the logging Module
import logging
logging.basicConfig(level=logging.INFO, filename='mylog.log')
logging.info('Starting program')
logging.info('Trying to divide 1 by 0')
print 1 / 0
logging.info('The division succeeded')
logging.info('Ending program')
Running that program would result in the following log file (called mylog.log):
INFO:root:Starting program
INFO:root:Trying to divide 1 by 0
As you can see, nothing is logged after trying to divide 1 by 0 because this error effectively
kills the program. Because this is such a simple error, you can tell what is wrong by the exception
traceback that prints as the program crashes. However, the most difficult type of bug to track
down is one that doesn’t stop your program but simply makes it behave strangely. Examining
a detailed log file may help you find out what’s going on.