The next two sections will take a quick tour through sys and os, before this chapter moves on to larger system programming concepts. As I'm not going to demonstrate every item in every built-in module, the first thing I want to do is show you how to get more details on your own. Officially, this task also serves as an excuse for introducing a few core system scripting concepts -- along the way, we'll code a first script to format documentation.
2.3.1 Python System Modules
Most system-level interfaces in Python are shipped in just two modules: sys and os. That's somewhat oversimplified; other standard modules belong to this domain too (e.g., glob, socket, thread, time, fcntl), and some built-in functions are really system interfaces as well (e.g., open). But sys and os together form the core of Python's system tools arsenal.
In principle at least, sys exports components related to the Python interpreter itself (e.g., the module search path), and os contains variables and functions that map to the operating system on which Python is run. In practice, this distinction may not always seem clear-cut (e.g., the standard input and output streams show up in sys, but they are at least arguably tied to operating system paradigms). The good news is that you'll soon use the tools in these modules so often that their locations will be permanently stamped on your memory.
 They may also work their way into your subconscious. Python newcomers sometimes appear on Internet discussion forums to express joy after "dreaming in Python" for the first time. All possible Freudian interpretations aside, it's not bad as dream motifs go; after all, there are worse languages to dream in.
The os module also attempts to provide a portable programming interface to the underlying operating system -- its functions may be implemented differently on different platforms, but they look the same everywhere to Python scripts. In addition, the os module exports a nested submodule, os.path, that provides a portable interface to file and directory processing tools.
2.3.2 Module Documentation Sources
As you can probably deduce from the preceding paragraphs, learning to write system scripts in Python is mostly a matter of learning about Python's system modules. Luckily, there are a variety of information sources to make this task easier -- from module attributes to published references and books.
For instance, if you want to know everything that a built-in module exports, you can either read its library manual entry, study its source code (Python is open source software, after all), or fetch its attribute list and documentation string interactively. Let's import sys and see what it's got:
C:...PP2ESystem> python >>> import sys >>> dir(sys) ['__doc__', '__name__', '__stderr__', '__stdin__', '__stdout__', 'argv', 'builtin_module_names', 'copyright', 'dllhandle', 'exc_info', 'exc_type', 'exec_prefix', 'executable', 'exit', 'getrefcount', 'hexversion', 'maxint', 'modules', 'path', 'platform', 'prefix', 'ps1', 'ps2', 'setcheckinterval', 'setprofile', 'settrace', 'stderr', 'stdin', 'stdout', 'version', 'winver']
The dir function simply returns a list containing the string names of all the attributes in any object with attributes; it's a handy memory-jogger for modules at the interactive prompt. For example, we know there is something called sys.version, because the name version came back in the dir result. If that's not enough, we can always consult the __doc__ string of built-in modules:
>>> sys.__doc__ ... ...lots of text deleted here... ... count for an object (plus one :-) 12setcheckinterval( ) -- control how often the interpreter checks for events 12setprofile( ) -- set the global profiling function 12settrace( ) -- set the global debug tracing function 12"
2.3.3 Paging Documentation Strings
The __doc__ built-in attribute usually contains a string of documentation, but may look a bit weird when printed -- it's one long string with embedded line-feed characters that print as