# 6.1 Standard Module `os`

This module provides a more portable way of using operating system (OS) dependent functionality than importing an OS dependent built-in module like `posix`.

When the optional built-in module `posix` is available, this module exports the same functions and data as `posix`; otherwise, it searches for an OS dependent built-in module like `mac` and exports the same functions and data as found there. The design of all Python's built-in OS dependent modules is such that as long as the same functionality is available, it uses the same interface; e.g., the function `os.stat(file)` returns stat info about a file in a format compatible with the POSIX interface.

Extensions peculiar to a particular OS are also available through the `os` module, but using them is of course a threat to portability!

Note that after the first time `os` is imported, there is no performance penalty in using functions from `os` instead of directly from the OS dependent built-in module, so there should be no reason not to use `os`!

In addition to whatever the correct OS dependent module exports, the following variables and functions are always exported by `os`:

`name`
  The name of the OS dependent module imported. The following names have currently been registered: `'posix'`, `'nt'`, `'dos'`, `'mac'`.

`path`
  The corresponding OS dependent standard module for pathname operations, e.g., `posixpath` or `macpath`. Thus, (given the proper imports), `os.path.split(file)` is equivalent to but more portable than `posixpath.split(file)`.

`curdir`
  The constant string used by the OS to refer to the current directory, e.g. `'.'` for POSIX or `':'` for the Mac.

`pardir`
  The constant string used by the OS to refer to the parent directory, e.g. `'..'` for POSIX or `'::'` for the Mac.

`sep`
  The character used by the OS to separate pathname components, e.g. `'/'` for POSIX or `':'` for the Mac. Note that knowing this is not sufficient to be able to parse or concatenate pathnames--better use `os.path.split()` and `os.path.join()`--but it is occasionally useful.

`altsep`
  An alternative character used by the OS to separate pathname components, or `None` if only one separator character exists. This is set to `'/'` on DOS/Windows systems where `sep` is a backslash.

`pathsep`
  The character conventionally used by the OS to separate search patch components (as in `$PATH`), e.g. `':'` for POSIX or `';'` for MS-DOS.

`defpath`
  The default search path used by `os.exec*p*()` if the environment doesn't have a `'PATH'` key.

`execl` (path, arg0, arg1, ...)
  This is equivalent to `os.execv(path, (arg0, arg1, ...))`.

`execle` (path, arg0, arg1, ..., env)
  This is equivalent to `os.execve(path, (arg0, arg1, ...), env)`.

`execlp` (path, arg0, arg1, ...)
  This is equivalent to `os.execvp(path, (arg0, arg1, ...))`.

`execvp` (path, args)
  This is like `os.execv(path, args)` but duplicates the shell's actions in searching for an executable file in a list of directories. The directory list is obtained from `os.environ['PATH']`.

`execvpe` (path, args, env)
  This is a cross between `os.execve()` and `os.execvp()`. The directory list is obtained from `env['PATH']`.

(The functions `os.execv()` and `execve()` are not documented here, since they are implemented by the OS dependent module. If the OS dependent module doesn't define either of these, the functions that rely on it will raise an exception. They are documented in the section on module `posix`, together with all other functions that `os` imports from the OS dependent module.)