Beginning Python - From Novice to Professional

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CHAPTER 10 ■ BATTERIES INCLUDED 217 Table 10-3. Some Important Functions and Variables in the os Module Function/Variable environ system(command) sep pathsep linesep urandom(n) Description Mapping with environment variables Executes an OS command in a subshell Separator used in paths Separator to separate paths Line separator ('\n', '\r', or '\r\n') Returns n bytes of cryptographically strong random data ■Tip A useful function for traversing directories is os.walk. Check it out in the Python Library Reference. The mapping os.environ contains environment variables described earlier in this chapter. For example, to access the environment variable PYTHONPATH, you would use the expression os.environ['PYTHONPATH']. This mapping can also be used to change environment variables, although not all platforms support this. The function os.system is used to run external programs. There are other functions for executing external programs, including execv, which exits the Python interpreter, yielding control to the executed program, and popen, which creates a file-like connection to the program. For more information about these functions, consult the standard library documentation. ■Tip The subprocess module is a recent addition to Python, which collects the functionality of the os.system, execv, and popen functions. The module variable os.sep is a separator used in pathnames. The standard separator in UNIX is '/', the standard in Windows is '\\' (the Python syntax for a single backslash), and in Mac OS, it is ':'. (On some platforms, os.altsep contains an alternate path separator, such as '/' in Windows.) You use os.pathsep when grouping several paths, as in PYTHONPATH. The pathsep is used to separate the pathnames: ':' in UNIX, ';' in Windows, and '::' in Mac OS. The module variable os.linesep is the line separator string used in text files. In UNIX this is a single newline character ('\n'), in Mac OS it’s a single carriage return character ('\r'), and in Windows it’s the combination of a carriage return and a newline ('\r\n'). The urandom function uses a system-dependent source of “real” (or, at least, cryptographically strong) randomness. If your platform doesn’t support it, you’ll get a NotImplementedError.
218 CHAPTER 10 ■ BATTERIES INCLUDED Example Starting a Web browser. The system command can be used to execute any external program, which is very useful in environments such as UNIX where you can execute programs (or commands) from the command line to list the contents of a directory, send e-mail, and so on. But it can be useful for starting programs with graphical user interfaces, too—such as a Web browser. In UNIX, you can do the following (provided that you have a browser at /usr/ bin/firefox): os.system('/usr/bin/firefox') A Windows version would be (again use the path of a browser you have installed) os.system(r'c:\"Program Files"\"Mozilla Firefox"\firefox.exe') Note that I’ve been careful about enclosing Program Files and Mozilla Firefox in quotes; otherwise DOS (which handles the command) balks at the whitespace. (This may be important for directories in your PYTHONPATH as well.) Note also that you have to use backslashes here because DOS gets confused by forward slashes. If you run this, you will notice that the browser tries to open a Web site named Files"\Mozilla...—the part of the command after the whitespace. Also, if you try to run this from IDLE, a DOS window appears, but the browser doesn’t start until you close that DOS window. All in all, not exactly ideal behavior. Another function that suits the job better is the Windows-specific function os.startfile: os.startfile(r' c:\Program Files\Mozilla Firefox\firefox.exe') As you can see, os.startfile accepts a plain path, even if it contains whitespace. (That is, don’t enclose “Program Files” in quotes as in the os.system example.) Note that in Windows, your Python program keeps on running after the external program has been started by os.system (or os.startfile), whereas in UNIX, your Python program waits for the os.system command to finish. A BETTER SOLUTION: WEBBROWSER The os.system function is useful for a lot of things, but for the specific task of launching a Web browser there’s an even better solution: the webbrowser module. It contains a function called open that lets you automatically launch a Web browser to open the given URL. For example, if you want your program to open the Python Web site in a Web browser (either starting a new browser or using one that is already running), you simply use import webbrowser webbrowser.open('http://www.python.org') and the page should pop up. Pretty nifty, huh?
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Beginning Python From Novice to Pro
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572 APPENDIX C ■ ONLINE RESOURCES
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Index ■Symbols - operator 558 !=
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