Python Programming for Beginners
Despite what assembly code and C coders might tell us, high-level languages do have their place in every programmer's toolbox, and some of them are much more than a computer-science curiosity. Out of the many high-level languages we can choose from today, Python seems to be the most interesting for those who want to learn something new and do real work at the same time. Its no-nonsense implementation of object-oriented programming and its clean and easy-to-understand syntax make it a language that is fun to learn and use, which is not something we can say about most other languages.
In this tutorial, you will learn how to write applications that use command-line options, read and write to pipes, access environment variables, handle interrupts, read from and write to files, create temporary files and write to system logs. In other words, you will find recipes for writing real applications instead of the old boring Hello, World! stuff.
To begin, if you have not installed the Python interpreter on your system, now is the time. To make that step easier, install the latest Python distribution using packages compatible with your Linux distribution. rpm, deb and tgz are also available on your Linux CD-ROM or on-line. If you follow standard installation procedures, you should not have any problems.
Next, read the excellent Python Tutorial written by Guido van Rossum, creator of the Python programming language. This tutorial is part of the official Python documentation, and you can find it in either the /usr/doc/python-docs-1.5.2 or /usr/local/doc/python-docs-1.5.2 catalog. It may be delivered in the raw LaTeX format, which must be processed first; if you don't know how to do this, go to http://www.python.org/doc/ to download it in an alternative format.
I also recommend that you have the Python Library Reference handy; you might want it when the explanations given here do not meet your needs. You can find it in the same places as the Python Tutorial.
Creating scripts can be done using your favorite text editor as long as it saves text in plain ASCII format and does not automatically insert line breaks when the line is longer than the width of the editor's window.
Always begin your scripts with either
#! /usr/bin/pythonIf the access path to the python binary on your system is different, change that line, leaving the first two characters (#!) intact. Be sure this line is truly the first line in your script, not just the first non-blank line—it will save you a lot of frustration.
Use chmod to set the file permissions on your script to make it executable. If the script is for you alone, type chmod 0700 scriptfilename.py; if you want to share it with others in your group but not let them edit it, use 0750 as the chmod value; if you want to give access to everyone else, use the value 0755. For help with the chmod command, type man chmod.
Command-line options and arguments come in handy when we want to tell our scripts how to behave or pass some arguments (file names, directory names, user names, etc.) to them. All programs can read these options and arguments if they want, and your Python scripts are no different.
Implementing appropriate handlers boils down to reading the argv list and checking for the options and arguments you want your script to recognize. There are a few ways to do this. Listing 1 is a simple option handler that recognizes common -h, -help and --help options, and when they are found, it exits immediately after displaying the help message.
Copy and save this script as help.py, make it executable with the chmod 0755 help.py command, and run it several times, specifying different options, both recognized by the handler and not; e.g. with one of the options, you will see this message: ./help.py -h or ./help.py -o. If the option handler does recognize one of the options, you will see this message:
help.py—does nothing useful (yet) options: -h, -help, or --help—display this help Copyright (c) Jacek Artymiak, 2000
If you invoke help.py with an option it does not recognize, or without any options at all, it will display the “I don't recognize this option” message.
Note that we need to import the sys module before we can check the contents of the argv list and before we can call the exit function. The sys.exit statement is a safety feature which prevents further program execution when one of the help options is found inside the argv list. This ensures that users don't do something dangerous before reading the help messages (for which they wouldn't have a need otherwise).
The simple help option handler described above works quite well and you can duplicate and change it to recognize additional options, but that is not the most efficient way to recognize multiple options with or without arguments. The “proper” way to do it is to use the getopt module, which converts options and arguments into a nice list of tuples. Listing 2 shows how it works.
Copy this script, save it as options.py and make it executable. As you can see, it uses two modules: sys and getopt which are imported right at the beginning. Then we define a simple function that displays the help message whenever something goes wrong.
The actual processing of command-line arguments begins with the try statement, where we are testing the list of command-line options and arguments (sys.argv) for errors defined as unknown options or missing arguments; if they are detected, the script will display an error message and exit immediately (see the except statement group). When no errors have been detected, our script splits the list of options and their arguments into tuples in the options list and begins parsing them by executing a series of loops, each searching for one option and its expected arguments.
The getopt.getopt function generates two lists in our sample script: options which contains options and their arguments; and xarguments which contains arguments not related to any of the options. We can safely ignore them in most cases.
To recognize short (one-letter such as -h) and long (those prefixed with --) options, getopt.getopt uses two separate arguments. The list of short options contains all of them listed in a single string, e.g., getopt.getopt(sys.argv, 'ahoinmdwq'). It is possible to specify, in that string, options that absolutely require an argument to follow them immediately (e.g., -vfilename) or after a space (e.g., -v filename). This is done by inserting a colon (:) after the option, like this: getopt.getopt(sys.argv, 'ahoiv:emwn'). However, this creates a silly problem that may cause some confusion and unnecessarily waste your time; if the user forgets to specify the argument for the option that requires it, the option that follows it becomes its argument. Consider this example:
script.py -v -h
If you put v: in the short option string argument of the getopt.getopt function, option -h will be treated as the argument of option -v. This is a nuisance and makes parsing of the list of tuples option, argument much more difficult. The solution to this problem is simple: don't use the colon, but check the second item of the tuple that contains the option (first item of the analyzed tuple) which requires an argument. If it's empty, report an error, like the -a option handler.
Long options prefixed with -- must be listed as a separate argument to the getopt.getopt, e.g., getopt.getopt(sys.argv, 'ah', ['view', 'file=']). They can be serviced by the same handler as short options.
What you do after locating options given by the user is up to you. Listing 2 can be used as a template for your scripts.
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