Beginning Python - From Novice to Professional

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.