Beginning Python - From Novice to Professional

216 CHAPTER 10 ■ BATTERIES INCLUDED
The module variable sys.platform (a string) is simply the name of the “platform” the
interpreter is running on. This may be either a name indicating an operating system (such as
sunos5 or win32) or it may indicate some other kind of platform, such as a Java virtual machine
(for example, java1.4.0) if you’re running Jython.
The module variables sys.stdin, sys.stdout, and sys.stderr are file-like stream objects.
They represent the standard UNIX concepts of standard input, standard output, and standard
error. To put it simply, sys.stdin is where Python gets its input (used in the functions input
and raw_input, for example), and sys.stdout is where it prints to. You learn more about files
(and these three streams) in Chapter 11.
Example
Printing the arguments in reverse order. When you call a Python script from the command line, you may add
some arguments after it—the so-called command-line arguments. These will then be placed in the list sys.argv,
with the name of the Python script as sys.argv[0]. Printing these out in reverse order is pretty simple, as you can
see in Listing 10-5.
Listing 10-5. Reversing and Printing Command-Line Arguments
# reverseargs.py
import sys
args = sys.argv[1:]
args.reverse()
print ' '.join(args)
As you can see, I make a copy of sys.argv. You can modify the original, but in general it’s safer not to because
other parts of the program may also rely on sys.argv containing the original arguments. Notice also that I skip the
first element of sys.argv—the name of the script. I reverse the list with args.reverse(), but I can’t print the
result of that operation. It is an in-place modification that returns None. Finally, to make the output prettier, I use the
join string method. Let’s try the result (assuming a UNIX shell here, but it will work equally well at an MS-DOS
prompt, for example):
$ python reverseargs.py this is a test
test a is this
os
The os module gives you access to several operating system services. The os module is extensive,
and only a few of the most useful functions and variables are described in Table 10-3. In addition to
these, os and its submodule os.path contain several functions to examine, construct, and remove
directories and files. For more information about this functionality, see the standard library
documentation.