Beginning Python - From Novice to Professional

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CHAPTER 19 ■ PLAYFUL PROGRAMMING 385 For more information on the features of the ConfigParser module, you should consult the library documentation (http://python.org/doc/lib/module-ConfigParser.html). Listing 19-1. A Simple Configuration File [numbers] pi: 3.1415926535897931 [messages] greeting: Welcome to the area calculation program! question: Please enter the radius: result_message: The area is Listing 19-2. A Program Using ConfigParser from ConfigParser import ConfigParser config = ConfigParser() # Read the configuration file: config.read('c:/python/config.txt') # Print out an initial greeting; # 'messages' is the section to look in: print config.get('messages', 'greeting') # Read in the radius, using a question from the config file: radius = input(config.get('messages', 'question') + ' ') # Print a result message from the config file; # end with a comma to stay on same line: print config.get('messages', 'result_message'), # getfloat() converts the config value to a float: print config.getfloat('numbers', 'pi') * radius**2 I won’t go into much detail about configuration in the following projects, but I suggest you think about making your programs highly configurable. That way, the user can adapt the program to his or her taste, which can make using it more pleasurable. After all, one of the main frustrations of using software is that you can’t make it behave the way you want it to. Logging Somewhat related to testing (discussed in Chapter 16), and quite useful when furiously reworking the innards of a program, logging can certainly help you discover problems and bugs. Logging is basically collecting data about your program as it runs, so you can examine it afterward (or as
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.
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Beginning Python From Novice to Pro
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viii ■CONTENTS Strings . . . . .
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About the Technical Reviewer ■JER
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Introduction A C program is like a
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256 CHAPTER 11 ■ FILES AND STUFF
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270 CHAPTER 12 ■ GRAPHICAL USER I
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298 CHAPTER 14 ■ NETWORK PROGRAMM
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APPENDIX A ■ ■ ■ The Short Ve
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572 APPENDIX C ■ ONLINE RESOURCES
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Index ■Symbols - operator 558 !=
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