15.1. "os" — Miscellaneous operating system interfaces
******************************************************

This module provides a portable way of using operating system
dependent functionality.  If you just want to read or write a file see
"open()", if you want to manipulate paths, see the "os.path" module,
and if you want to read all the lines in all the files on the command
line see the "fileinput" module.  For creating temporary files and
directories see the "tempfile" module, and for high-level file and
directory handling see the "shutil" module.

Notes on the availability of these functions:

* The design of all built-in operating system dependent modules of
  Python is such that as long as the same functionality is available,
  it uses the same interface; for example, the function
  "os.stat(path)" returns stat information about *path* in the same
  format (which happens to have originated with the POSIX interface).

* Extensions peculiar to a particular operating system are also
  available through the "os" module, but using them is of course a
  threat to portability.

* An “Availability: Unix” note means that this function is commonly
  found on Unix systems.  It does not make any claims about its
  existence on a specific operating system.

* If not separately noted, all functions that claim “Availability:
  Unix” are supported on Mac OS X, which builds on a Unix core.

Note: All functions in this module raise "OSError" in the case of
  invalid or inaccessible file names and paths, or other arguments
  that have the correct type, but are not accepted by the operating
  system.

exception os.error

   An alias for the built-in "OSError" exception.

os.name

   The name of the operating system dependent module imported.  The
   following names have currently been registered: "'posix'", "'nt'",
   "'os2'", "'ce'", "'java'", "'riscos'".

   See also: "sys.platform" has a finer granularity.  "os.uname()"
     gives system-dependent version information.

     The "platform" module provides detailed checks for the system’s
     identity.


15.1.1. Process Parameters
==========================

These functions and data items provide information and operate on the
current process and user.

os.environ

   A *mapping* object representing the string environment. For
   example, "environ['HOME']" is the pathname of your home directory
   (on some platforms), and is equivalent to "getenv("HOME")" in C.

   This mapping is captured the first time the "os" module is
   imported, typically during Python startup as part of processing
   "site.py".  Changes to the environment made after this time are not
   reflected in "os.environ", except for changes made by modifying
   "os.environ" directly.

   If the platform supports the "putenv()" function, this mapping may
   be used to modify the environment as well as query the environment.
   "putenv()" will be called automatically when the mapping is
   modified.

   Note: Calling "putenv()" directly does not change "os.environ",
     so it’s better to modify "os.environ".

   Note: On some platforms, including FreeBSD and Mac OS X, setting
     "environ" may cause memory leaks.  Refer to the system
     documentation for "putenv()".

   If "putenv()" is not provided, a modified copy of this mapping  may
   be passed to the appropriate process-creation functions to cause
   child processes to use a modified environment.

   If the platform supports the "unsetenv()" function, you can delete
   items in this mapping to unset environment variables. "unsetenv()"
   will be called automatically when an item is deleted from
   "os.environ", and when one of the "pop()" or "clear()" methods is
   called.

   Changed in version 2.6: Also unset environment variables when
   calling "os.environ.clear()" and "os.environ.pop()".

os.chdir(path)
os.fchdir(fd)
os.getcwd()

   These functions are described in Files and Directories.

os.ctermid()

   Return the filename corresponding to the controlling terminal of
   the process.

   Availability: Unix.

os.getegid()

   Return the effective group id of the current process.  This
   corresponds to the “set id” bit on the file being executed in the
   current process.

   Availability: Unix.

os.geteuid()

   Return the current process’s effective user id.

   Availability: Unix.

os.getgid()

   Return the real group id of the current process.

   Availability: Unix.

os.getgroups()

   Return list of supplemental group ids associated with the current
   process.

   Availability: Unix.

   Note: On Mac OS X, "getgroups()" behavior differs somewhat from
     other Unix platforms. If the Python interpreter was built with a
     deployment target of "10.5" or earlier, "getgroups()" returns the
     list of effective group ids associated with the current user
     process; this list is limited to a system-defined number of
     entries, typically 16, and may be modified by calls to
     "setgroups()" if suitably privileged. If built with a deployment
     target greater than "10.5", "getgroups()" returns the current
     group access list for the user associated with the effective user
     id of the process; the group access list may change over the
     lifetime of the process, it is not affected by calls to
     "setgroups()", and its length is not limited to 16.  The
     deployment target value, "MACOSX_DEPLOYMENT_TARGET", can be
     obtained with "sysconfig.get_config_var()".

os.initgroups(username, gid)

   Call the system initgroups() to initialize the group access list
   with all of the groups of which the specified username is a member,
   plus the specified group id.

   Availability: Unix.

   New in version 2.7.

os.getlogin()

   Return the name of the user logged in on the controlling terminal
   of the process.  For most purposes, it is more useful to use the
   environment variable "LOGNAME" to find out who the user is, or
   "pwd.getpwuid(os.getuid())[0]" to get the login name of the
   process’s real user id.

   Availability: Unix.

os.getpgid(pid)

   Return the process group id of the process with process id *pid*.
   If *pid* is 0, the process group id of the current process is
   returned.

   Availability: Unix.

   New in version 2.3.

os.getpgrp()

   Return the id of the current process group.

   Availability: Unix.

os.getpid()

   Return the current process id.

   Availability: Unix, Windows.

os.getppid()

   Return the parent’s process id.

   Availability: Unix.

os.getresuid()

   Return a tuple (ruid, euid, suid) denoting the current process’s
   real, effective, and saved user ids.

   Availability: Unix.

   New in version 2.7.

os.getresgid()

   Return a tuple (rgid, egid, sgid) denoting the current process’s
   real, effective, and saved group ids.

   Availability: Unix.

   New in version 2.7.

os.getuid()

   Return the current process’s real user id.

   Availability: Unix.

os.getenv(varname[, value])

   Return the value of the environment variable *varname* if it
   exists, or *value* if it doesn’t.  *value* defaults to "None".

   Availability: most flavors of Unix, Windows.

os.putenv(varname, value)

   Set the environment variable named *varname* to the string *value*.
   Such changes to the environment affect subprocesses started with
   "os.system()", "popen()" or "fork()" and "execv()".

   Availability: most flavors of Unix, Windows.

   Note: On some platforms, including FreeBSD and Mac OS X, setting
     "environ" may cause memory leaks. Refer to the system
     documentation for putenv.

   When "putenv()" is supported, assignments to items in "os.environ"
   are automatically translated into corresponding calls to
   "putenv()"; however, calls to "putenv()" don’t update "os.environ",
   so it is actually preferable to assign to items of "os.environ".

os.setegid(egid)

   Set the current process’s effective group id.

   Availability: Unix.

os.seteuid(euid)

   Set the current process’s effective user id.

   Availability: Unix.

os.setgid(gid)

   Set the current process’ group id.

   Availability: Unix.

os.setgroups(groups)

   Set the list of supplemental group ids associated with the current
   process to *groups*. *groups* must be a sequence, and each element
   must be an integer identifying a group. This operation is typically
   available only to the superuser.

   Availability: Unix.

   New in version 2.2.

   Note: On Mac OS X, the length of *groups* may not exceed the
     system- defined maximum number of effective group ids, typically
     16. See the documentation for "getgroups()" for cases where it
     may not return the same group list set by calling setgroups().

os.setpgrp()

   Call the system call "setpgrp()" or "setpgrp(0, 0)" depending on
   which version is implemented (if any).  See the Unix manual for the
   semantics.

   Availability: Unix.

os.setpgid(pid, pgrp)

   Call the system call "setpgid()" to set the process group id of the
   process with id *pid* to the process group with id *pgrp*.  See the
   Unix manual for the semantics.

   Availability: Unix.

os.setregid(rgid, egid)

   Set the current process’s real and effective group ids.

   Availability: Unix.

os.setresgid(rgid, egid, sgid)

   Set the current process’s real, effective, and saved group ids.

   Availability: Unix.

   New in version 2.7.

os.setresuid(ruid, euid, suid)

   Set the current process’s real, effective, and saved user ids.

   Availability: Unix.

   New in version 2.7.

os.setreuid(ruid, euid)

   Set the current process’s real and effective user ids.

   Availability: Unix.

os.getsid(pid)

   Call the system call "getsid()".  See the Unix manual for the
   semantics.

   Availability: Unix.

   New in version 2.4.

os.setsid()

   Call the system call "setsid()".  See the Unix manual for the
   semantics.

   Availability: Unix.

os.setuid(uid)

   Set the current process’s user id.

   Availability: Unix.

os.strerror(code)

   Return the error message corresponding to the error code in *code*.
   On platforms where "strerror()" returns "NULL" when given an
   unknown error number, "ValueError" is raised.

   Availability: Unix, Windows.

os.umask(mask)

   Set the current numeric umask and return the previous umask.

   Availability: Unix, Windows.

os.uname()

   Return a 5-tuple containing information identifying the current
   operating system.  The tuple contains 5 strings: "(sysname,
   nodename, release, version, machine)".  Some systems truncate the
   nodename to 8 characters or to the leading component; a better way
   to get the hostname is "socket.gethostname()"  or even
   "socket.gethostbyaddr(socket.gethostname())".

   Availability: recent flavors of Unix.

os.unsetenv(varname)

   Unset (delete) the environment variable named *varname*. Such
   changes to the environment affect subprocesses started with
   "os.system()", "popen()" or "fork()" and "execv()".

   When "unsetenv()" is supported, deletion of items in "os.environ"
   is automatically translated into a corresponding call to
   "unsetenv()"; however, calls to "unsetenv()" don’t update
   "os.environ", so it is actually preferable to delete items of
   "os.environ".

   Availability: most flavors of Unix, Windows.


15.1.2. File Object Creation
============================

These functions create new file objects. (See also "open()".)

os.fdopen(fd[, mode[, bufsize]])

   Return an open file object connected to the file descriptor *fd*.
   The *mode* and *bufsize* arguments have the same meaning as the
   corresponding arguments to the built-in "open()" function.  If
   "fdopen()" raises an exception, it leaves *fd* untouched
   (unclosed).

   Availability: Unix, Windows.

   Changed in version 2.3: When specified, the *mode* argument must
   now start with one of the letters "'r'", "'w'", or "'a'", otherwise
   a "ValueError" is raised.

   Changed in version 2.5: On Unix, when the *mode* argument starts
   with "'a'", the *O_APPEND* flag is set on the file descriptor
   (which the "fdopen()" implementation already does on most
   platforms).

os.popen(command[, mode[, bufsize]])

   Open a pipe to or from *command*.  The return value is an open file
   object connected to the pipe, which can be read or written
   depending on whether *mode* is "'r'" (default) or "'w'". The
   *bufsize* argument has the same meaning as the corresponding
   argument to the built-in "open()" function.  The exit status of the
   command (encoded in the format specified for "wait()") is available
   as the return value of the "close()" method of the file object,
   except that when the exit status is zero (termination without
   errors), "None" is returned.

   Availability: Unix, Windows.

   Deprecated since version 2.6: This function is obsolete.  Use the
   "subprocess" module.  Check especially the Replacing Older
   Functions with the subprocess Module section.

   Changed in version 2.0: This function worked unreliably under
   Windows in earlier versions of Python. This was due to the use of
   the "_popen()" function from the libraries provided with Windows.
   Newer versions of Python do not use the broken implementation from
   the Windows libraries.

os.tmpfile()

   Return a new file object opened in update mode ("w+b").  The file
   has no directory entries associated with it and will be
   automatically deleted once there are no file descriptors for the
   file.

   Availability: Unix, Windows.

There are a number of different "popen*()" functions that provide
slightly different ways to create subprocesses.

Deprecated since version 2.6: All of the "popen*()" functions are
obsolete. Use the "subprocess" module.

For each of the "popen*()" variants, if *bufsize* is specified, it
specifies the buffer size for the I/O pipes. *mode*, if provided,
should be the string "'b'" or "'t'"; on Windows this is needed to
determine whether the file objects should be opened in binary or text
mode.  The default value for *mode* is "'t'".

Also, for each of these variants, on Unix, *cmd* may be a sequence, in
which case arguments will be passed directly to the program without
shell intervention (as with "os.spawnv()"). If *cmd* is a string it
will be passed to the shell (as with "os.system()").

These methods do not make it possible to retrieve the exit status from
the child processes.  The only way to control the input and output
streams and also retrieve the return codes is to use the "subprocess"
module; these are only available on Unix.

For a discussion of possible deadlock conditions related to the use of
these functions, see Flow Control Issues.

os.popen2(cmd[, mode[, bufsize]])

   Execute *cmd* as a sub-process and return the file objects
   "(child_stdin, child_stdout)".

   Deprecated since version 2.6: This function is obsolete.  Use the
   "subprocess" module.  Check especially the Replacing Older
   Functions with the subprocess Module section.

   Availability: Unix, Windows.

   New in version 2.0.

os.popen3(cmd[, mode[, bufsize]])

   Execute *cmd* as a sub-process and return the file objects
   "(child_stdin, child_stdout, child_stderr)".

   Deprecated since version 2.6: This function is obsolete.  Use the
   "subprocess" module.  Check especially the Replacing Older
   Functions with the subprocess Module section.

   Availability: Unix, Windows.

   New in version 2.0.

os.popen4(cmd[, mode[, bufsize]])

   Execute *cmd* as a sub-process and return the file objects
   "(child_stdin, child_stdout_and_stderr)".

   Deprecated since version 2.6: This function is obsolete.  Use the
   "subprocess" module.  Check especially the Replacing Older
   Functions with the subprocess Module section.

   Availability: Unix, Windows.

   New in version 2.0.

(Note that "child_stdin, child_stdout, and child_stderr" are named
from the point of view of the child process, so *child_stdin* is the
child’s standard input.)

This functionality is also available in the "popen2" module using
functions of the same names, but the return values of those functions
have a different order.


15.1.3. File Descriptor Operations
==================================

These functions operate on I/O streams referenced using file
descriptors.

File descriptors are small integers corresponding to a file that has
been opened by the current process.  For example, standard input is
usually file descriptor 0, standard output is 1, and standard error is
2.  Further files opened by a process will then be assigned 3, 4, 5,
and so forth.  The name “file descriptor” is slightly deceptive; on
Unix platforms, sockets and pipes are also referenced by file
descriptors.

The "fileno()" method can be used to obtain the file descriptor
associated with a file object when required.  Note that using the file
descriptor directly will bypass the file object methods, ignoring
aspects such as internal buffering of data.

os.close(fd)

   Close file descriptor *fd*.

   Availability: Unix, Windows.

   Note: This function is intended for low-level I/O and must be
     applied to a file descriptor as returned by "os.open()" or
     "pipe()".  To close a “file object” returned by the built-in
     function "open()" or by "popen()" or "fdopen()", use its
     "close()" method.

os.closerange(fd_low, fd_high)

   Close all file descriptors from *fd_low* (inclusive) to *fd_high*
   (exclusive), ignoring errors. Equivalent to:

      for fd in xrange(fd_low, fd_high):
          try:
              os.close(fd)
          except OSError:
              pass

   Availability: Unix, Windows.

   New in version 2.6.

os.dup(fd)

   Return a duplicate of file descriptor *fd*.

   Availability: Unix, Windows.

os.dup2(fd, fd2)

   Duplicate file descriptor *fd* to *fd2*, closing the latter first
   if necessary.

   Availability: Unix, Windows.

os.fchmod(fd, mode)

   Change the mode of the file given by *fd* to the numeric *mode*.
   See the docs for "chmod()" for possible values of *mode*.

   Availability: Unix.

   New in version 2.6.

os.fchown(fd, uid, gid)

   Change the owner and group id of the file given by *fd* to the
   numeric *uid* and *gid*.  To leave one of the ids unchanged, set it
   to -1.

   Availability: Unix.

   New in version 2.6.

os.fdatasync(fd)

   Force write of file with filedescriptor *fd* to disk. Does not
   force update of metadata.

   Availability: Unix.

   Note: This function is not available on MacOS.

os.fpathconf(fd, name)

   Return system configuration information relevant to an open file.
   *name* specifies the configuration value to retrieve; it may be a
   string which is the name of a defined system value; these names are
   specified in a number of standards (POSIX.1, Unix 95, Unix 98, and
   others).  Some platforms define additional names as well.  The
   names known to the host operating system are given in the
   "pathconf_names" dictionary.  For configuration variables not
   included in that mapping, passing an integer for *name* is also
   accepted.

   If *name* is a string and is not known, "ValueError" is raised.  If
   a specific value for *name* is not supported by the host system,
   even if it is included in "pathconf_names", an "OSError" is raised
   with "errno.EINVAL" for the error number.

   Availability: Unix.

os.fstat(fd)

   Return status for file descriptor *fd*, like "stat()".

   Availability: Unix, Windows.

os.fstatvfs(fd)

   Return information about the filesystem containing the file
   associated with file descriptor *fd*, like "statvfs()".

   Availability: Unix.

os.fsync(fd)

   Force write of file with filedescriptor *fd* to disk.  On Unix,
   this calls the native "fsync()" function; on Windows, the MS
   "_commit()" function.

   If you’re starting with a Python file object *f*, first do
   "f.flush()", and then do "os.fsync(f.fileno())", to ensure that all
   internal buffers associated with *f* are written to disk.

   Availability: Unix, and Windows starting in 2.2.3.

os.ftruncate(fd, length)

   Truncate the file corresponding to file descriptor *fd*, so that it
   is at most *length* bytes in size.

   Availability: Unix.

os.isatty(fd)

   Return "True" if the file descriptor *fd* is open and connected to
   a tty(-like) device, else "False".

os.lseek(fd, pos, how)

   Set the current position of file descriptor *fd* to position *pos*,
   modified by *how*: "SEEK_SET" or "0" to set the position relative
   to the beginning of the file; "SEEK_CUR" or "1" to set it relative
   to the current position; "SEEK_END" or "2" to set it relative to
   the end of the file. Return the new cursor position in bytes,
   starting from the beginning.

   Availability: Unix, Windows.

os.SEEK_SET
os.SEEK_CUR
os.SEEK_END

   Parameters to the "lseek()" function. Their values are 0, 1, and 2,
   respectively.

   Availability: Windows, Unix.

   New in version 2.5.

os.open(file, flags[, mode])

   Open the file *file* and set various flags according to *flags* and
   possibly its mode according to *mode*. The default *mode* is "0777"
   (octal), and the current umask value is first masked out.  Return
   the file descriptor for the newly opened file.

   For a description of the flag and mode values, see the C run-time
   documentation; flag constants (like "O_RDONLY" and "O_WRONLY") are
   defined in this module too (see open() flag constants).  In
   particular, on Windows adding "O_BINARY" is needed to open files in
   binary mode.

   Availability: Unix, Windows.

   Note: This function is intended for low-level I/O.  For normal
     usage, use the built-in function "open()", which returns a “file
     object” with "read()" and "write()" methods (and many more).  To
     wrap a file descriptor in a “file object”, use "fdopen()".

os.openpty()

   Open a new pseudo-terminal pair. Return a pair of file descriptors
   "(master, slave)" for the pty and the tty, respectively. For a
   (slightly) more portable approach, use the "pty" module.

   Availability: some flavors of Unix.

os.pipe()

   Create a pipe.  Return a pair of file descriptors "(r, w)" usable
   for reading and writing, respectively.

   Availability: Unix, Windows.

os.read(fd, n)

   Read at most *n* bytes from file descriptor *fd*. Return a string
   containing the bytes read.  If the end of the file referred to by
   *fd* has been reached, an empty string is returned.

   Availability: Unix, Windows.

   Note: This function is intended for low-level I/O and must be
     applied to a file descriptor as returned by "os.open()" or
     "pipe()".  To read a “file object” returned by the built-in
     function "open()" or by "popen()" or "fdopen()", or "sys.stdin",
     use its "read()" or "readline()" methods.

os.tcgetpgrp(fd)

   Return the process group associated with the terminal given by *fd*
   (an open file descriptor as returned by "os.open()").

   Availability: Unix.

os.tcsetpgrp(fd, pg)

   Set the process group associated with the terminal given by *fd*
   (an open file descriptor as returned by "os.open()") to *pg*.

   Availability: Unix.

os.ttyname(fd)

   Return a string which specifies the terminal device associated with
   file descriptor *fd*.  If *fd* is not associated with a terminal
   device, an exception is raised.

   Availability: Unix.

os.write(fd, str)

   Write the string *str* to file descriptor *fd*. Return the number
   of bytes actually written.

   Availability: Unix, Windows.

   Note: This function is intended for low-level I/O and must be
     applied to a file descriptor as returned by "os.open()" or
     "pipe()".  To write a “file object” returned by the built-in
     function "open()" or by "popen()" or "fdopen()", or "sys.stdout"
     or "sys.stderr", use its "write()" method.


15.1.3.1. "open()" flag constants
---------------------------------

The following constants are options for the *flags* parameter to the
"open()" function.  They can be combined using the bitwise OR operator
"|".  Some of them are not available on all platforms.  For
descriptions of their availability and use, consult the *open(2)*
manual page on Unix or the MSDN on Windows.

os.O_RDONLY
os.O_WRONLY
os.O_RDWR
os.O_APPEND
os.O_CREAT
os.O_EXCL
os.O_TRUNC

   The above constants are available on Unix and Windows.

os.O_DSYNC
os.O_RSYNC
os.O_SYNC
os.O_NDELAY
os.O_NONBLOCK
os.O_NOCTTY

   The above constants are only available on Unix.

os.O_BINARY
os.O_NOINHERIT
os.O_SHORT_LIVED
os.O_TEMPORARY
os.O_RANDOM
os.O_SEQUENTIAL
os.O_TEXT

   The above constants are only available on Windows.

os.O_ASYNC
os.O_DIRECT
os.O_DIRECTORY
os.O_NOFOLLOW
os.O_NOATIME
os.O_SHLOCK
os.O_EXLOCK

   The above constants are extensions and not present if they are not
   defined by the C library.


15.1.4. Files and Directories
=============================

os.access(path, mode)

   Use the real uid/gid to test for access to *path*.  Note that most
   operations will use the effective uid/gid, therefore this routine
   can be used in a suid/sgid environment to test if the invoking user
   has the specified access to *path*.  *mode* should be "F_OK" to
   test the existence of *path*, or it can be the inclusive OR of one
   or more of "R_OK", "W_OK", and "X_OK" to test permissions.  Return
   "True" if access is allowed, "False" if not. See the Unix man page
   *access(2)* for more information.

   Availability: Unix, Windows.

   Note: Using "access()" to check if a user is authorized to e.g.
     open a file before actually doing so using "open()" creates a
     security hole, because the user might exploit the short time
     interval between checking and opening the file to manipulate it.
     It’s preferable to use *EAFP* techniques. For example:

        if os.access("myfile", os.R_OK):
            with open("myfile") as fp:
                return fp.read()
        return "some default data"

     is better written as:

        try:
            fp = open("myfile")
        except IOError as e:
            if e.errno == errno.EACCES:
                return "some default data"
            # Not a permission error.
            raise
        else:
            with fp:
                return fp.read()

   Note: I/O operations may fail even when "access()" indicates that
     they would succeed, particularly for operations on network
     filesystems which may have permissions semantics beyond the usual
     POSIX permission-bit model.

os.F_OK

   Value to pass as the *mode* parameter of "access()" to test the
   existence of *path*.

os.R_OK

   Value to include in the *mode* parameter of "access()" to test the
   readability of *path*.

os.W_OK

   Value to include in the *mode* parameter of "access()" to test the
   writability of *path*.

os.X_OK

   Value to include in the *mode* parameter of "access()" to determine
   if *path* can be executed.

os.chdir(path)

   Change the current working directory to *path*.

   Availability: Unix, Windows.

os.fchdir(fd)

   Change the current working directory to the directory represented
   by the file descriptor *fd*.  The descriptor must refer to an
   opened directory, not an open file.

   Availability: Unix.

   New in version 2.3.

os.getcwd()

   Return a string representing the current working directory.

   Availability: Unix, Windows.

os.getcwdu()

   Return a Unicode object representing the current working directory.

   Availability: Unix, Windows.

   New in version 2.3.

os.chflags(path, flags)

   Set the flags of *path* to the numeric *flags*. *flags* may take a
   combination (bitwise OR) of the following values (as defined in the
   "stat" module):

   * "stat.UF_NODUMP"

   * "stat.UF_IMMUTABLE"

   * "stat.UF_APPEND"

   * "stat.UF_OPAQUE"

   * "stat.UF_NOUNLINK"

   * "stat.UF_COMPRESSED"

   * "stat.UF_HIDDEN"

   * "stat.SF_ARCHIVED"

   * "stat.SF_IMMUTABLE"

   * "stat.SF_APPEND"

   * "stat.SF_NOUNLINK"

   * "stat.SF_SNAPSHOT"

   Availability: Unix.

   New in version 2.6.

os.chroot(path)

   Change the root directory of the current process to *path*.
   Availability: Unix.

   New in version 2.2.

os.chmod(path, mode)

   Change the mode of *path* to the numeric *mode*. *mode* may take
   one of the following values (as defined in the "stat" module) or
   bitwise ORed combinations of them:

   * "stat.S_ISUID"

   * "stat.S_ISGID"

   * "stat.S_ENFMT"

   * "stat.S_ISVTX"

   * "stat.S_IREAD"

   * "stat.S_IWRITE"

   * "stat.S_IEXEC"

   * "stat.S_IRWXU"

   * "stat.S_IRUSR"

   * "stat.S_IWUSR"

   * "stat.S_IXUSR"

   * "stat.S_IRWXG"

   * "stat.S_IRGRP"

   * "stat.S_IWGRP"

   * "stat.S_IXGRP"

   * "stat.S_IRWXO"

   * "stat.S_IROTH"

   * "stat.S_IWOTH"

   * "stat.S_IXOTH"

   Availability: Unix, Windows.

   Note: Although Windows supports "chmod()", you can only  set the
     file’s read-only flag with it (via the "stat.S_IWRITE"  and
     "stat.S_IREAD" constants or a corresponding integer value).  All
     other bits are ignored.

os.chown(path, uid, gid)

   Change the owner and group id of *path* to the numeric *uid* and
   *gid*. To leave one of the ids unchanged, set it to -1.

   Availability: Unix.

os.lchflags(path, flags)

   Set the flags of *path* to the numeric *flags*, like "chflags()",
   but do not follow symbolic links.

   Availability: Unix.

   New in version 2.6.

os.lchmod(path, mode)

   Change the mode of *path* to the numeric *mode*. If path is a
   symlink, this affects the symlink rather than the target. See the
   docs for "chmod()" for possible values of *mode*.

   Availability: Unix.

   New in version 2.6.

os.lchown(path, uid, gid)

   Change the owner and group id of *path* to the numeric *uid* and
   *gid*. This function will not follow symbolic links.

   Availability: Unix.

   New in version 2.3.

os.link(source, link_name)

   Create a hard link pointing to *source* named *link_name*.

   Availability: Unix.

os.listdir(path)

   Return a list containing the names of the entries in the directory
   given by *path*.  The list is in arbitrary order.  It does not
   include the special entries "'.'" and "'..'" even if they are
   present in the directory.

   Availability: Unix, Windows.

   Changed in version 2.3: On Windows NT/2k/XP and Unix, if *path* is
   a Unicode object, the result will be a list of Unicode objects.
   Undecodable filenames will still be returned as string objects.

os.lstat(path)

   Perform the equivalent of an "lstat()" system call on the given
   path. Similar to "stat()", but does not follow symbolic links.  On
   platforms that do not support symbolic links, this is an alias for
   "stat()".

os.mkfifo(path[, mode])

   Create a FIFO (a named pipe) named *path* with numeric mode *mode*.
   The default *mode* is "0666" (octal).  The current umask value is
   first masked out from the mode.

   Availability: Unix.

   FIFOs are pipes that can be accessed like regular files.  FIFOs
   exist until they are deleted (for example with "os.unlink()").
   Generally, FIFOs are used as rendezvous between “client” and
   “server” type processes: the server opens the FIFO for reading, and
   the client opens it for writing.  Note that "mkfifo()" doesn’t open
   the FIFO — it just creates the rendezvous point.

os.mknod(filename[, mode=0600[, device=0]])

   Create a filesystem node (file, device special file or named pipe)
   named *filename*. *mode* specifies both the permissions to use and
   the type of node to be created, being combined (bitwise OR) with
   one of "stat.S_IFREG", "stat.S_IFCHR", "stat.S_IFBLK", and
   "stat.S_IFIFO" (those constants are available in "stat"). For
   "stat.S_IFCHR" and "stat.S_IFBLK", *device* defines the newly
   created device special file (probably using "os.makedev()"),
   otherwise it is ignored.

   New in version 2.3.

os.major(device)

   Extract the device major number from a raw device number (usually
   the "st_dev" or "st_rdev" field from "stat").

   New in version 2.3.

os.minor(device)

   Extract the device minor number from a raw device number (usually
   the "st_dev" or "st_rdev" field from "stat").

   New in version 2.3.

os.makedev(major, minor)

   Compose a raw device number from the major and minor device
   numbers.

   New in version 2.3.

os.mkdir(path[, mode])

   Create a directory named *path* with numeric mode *mode*. The
   default *mode* is "0777" (octal).  If the directory already exists,
   "OSError" is raised.

   On some systems, *mode* is ignored.  Where it is used, the current
   umask value is first masked out.  If bits other than the last 9
   (i.e. the last 3 digits of the octal representation of the *mode*)
   are set, their meaning is platform-dependent.  On some platforms,
   they are ignored and you should call "chmod()" explicitly to set
   them.

   It is also possible to create temporary directories; see the
   "tempfile" module’s "tempfile.mkdtemp()" function.

   Availability: Unix, Windows.

os.makedirs(path[, mode])

   Recursive directory creation function.  Like "mkdir()", but makes
   all intermediate-level directories needed to contain the leaf
   directory.  Raises an "error" exception if the leaf directory
   already exists or cannot be created.  The default *mode* is "0777"
   (octal).

   The *mode* parameter is passed to "mkdir()"; see the mkdir()
   description for how it is interpreted.

   Note: "makedirs()" will become confused if the path elements to
     create include "os.pardir".

   New in version 1.5.2.

   Changed in version 2.3: This function now handles UNC paths
   correctly.

os.pathconf(path, name)

   Return system configuration information relevant to a named file.
   *name* specifies the configuration value to retrieve; it may be a
   string which is the name of a defined system value; these names are
   specified in a number of standards (POSIX.1, Unix 95, Unix 98, and
   others).  Some platforms define additional names as well.  The
   names known to the host operating system are given in the
   "pathconf_names" dictionary.  For configuration variables not
   included in that mapping, passing an integer for *name* is also
   accepted.

   If *name* is a string and is not known, "ValueError" is raised.  If
   a specific value for *name* is not supported by the host system,
   even if it is included in "pathconf_names", an "OSError" is raised
   with "errno.EINVAL" for the error number.

   Availability: Unix.

os.pathconf_names

   Dictionary mapping names accepted by "pathconf()" and "fpathconf()"
   to the integer values defined for those names by the host operating
   system.  This can be used to determine the set of names known to
   the system. Availability: Unix.

os.readlink(path)

   Return a string representing the path to which the symbolic link
   points.  The result may be either an absolute or relative pathname;
   if it is relative, it may be converted to an absolute pathname
   using "os.path.join(os.path.dirname(path), result)".

   Changed in version 2.6: If the *path* is a Unicode object the
   result will also be a Unicode object.

   Availability: Unix.

os.remove(path)

   Remove (delete) the file *path*.  If *path* is a directory,
   "OSError" is raised; see "rmdir()" below to remove a directory.
   This is identical to the "unlink()" function documented below.  On
   Windows, attempting to remove a file that is in use causes an
   exception to be raised; on Unix, the directory entry is removed but
   the storage allocated to the file is not made available until the
   original file is no longer in use.

   Availability: Unix, Windows.

os.removedirs(path)

   Remove directories recursively.  Works like "rmdir()" except that,
   if the leaf directory is successfully removed, "removedirs()"
   tries to successively remove every parent directory mentioned in
   *path* until an error is raised (which is ignored, because it
   generally means that a parent directory is not empty). For example,
   "os.removedirs('foo/bar/baz')" will first remove the directory
   "'foo/bar/baz'", and then remove "'foo/bar'" and "'foo'" if they
   are empty. Raises "OSError" if the leaf directory could not be
   successfully removed.

   New in version 1.5.2.

os.rename(src, dst)

   Rename the file or directory *src* to *dst*.  If *dst* is a
   directory, "OSError" will be raised.  On Unix, if *dst* exists and
   is a file, it will be replaced silently if the user has permission.
   The operation may fail on some Unix flavors if *src* and *dst* are
   on different filesystems.  If successful, the renaming will be an
   atomic operation (this is a POSIX requirement).  On Windows, if
   *dst* already exists, "OSError" will be raised even if it is a
   file; there may be no way to implement an atomic rename when *dst*
   names an existing file.

   Availability: Unix, Windows.

os.renames(old, new)

   Recursive directory or file renaming function. Works like
   "rename()", except creation of any intermediate directories needed
   to make the new pathname good is attempted first. After the rename,
   directories corresponding to rightmost path segments of the old
   name will be pruned away using "removedirs()".

   New in version 1.5.2.

   Note: This function can fail with the new directory structure
     made if you lack permissions needed to remove the leaf directory
     or file.

os.rmdir(path)

   Remove (delete) the directory *path*.  Only works when the
   directory is empty, otherwise, "OSError" is raised.  In order to
   remove whole directory trees, "shutil.rmtree()" can be used.

   Availability: Unix, Windows.

os.stat(path)

   Perform the equivalent of a "stat()" system call on the given path.
   (This function follows symlinks; to stat a symlink use "lstat()".)

   The return value is an object whose attributes correspond to the
   members of the "stat" structure, namely:

   * "st_mode" - protection bits,

   * "st_ino" - inode number,

   * "st_dev" - device,

   * "st_nlink" - number of hard links,

   * "st_uid" - user id of owner,

   * "st_gid" - group id of owner,

   * "st_size" - size of file, in bytes,

   * "st_atime" - time of most recent access,

   * "st_mtime" - time of most recent content modification,

   * "st_ctime" - platform dependent; time of most recent metadata
     change on Unix, or the time of creation on Windows)

   Changed in version 2.3: If "stat_float_times()" returns "True", the
   time values are floats, measuring seconds. Fractions of a second
   may be reported if the system supports that. See
   "stat_float_times()" for further discussion.

   On some Unix systems (such as Linux), the following attributes may
   also be available:

   * "st_blocks" - number of 512-byte blocks allocated for file

   * "st_blksize" - filesystem blocksize for efficient file system
     I/O

   * "st_rdev" - type of device if an inode device

   * "st_flags" - user defined flags for file

   On other Unix systems (such as FreeBSD), the following attributes
   may be available (but may be only filled out if root tries to use
   them):

   * "st_gen" - file generation number

   * "st_birthtime" - time of file creation

   On RISCOS systems, the following attributes are also available:

   * "st_ftype" (file type)

   * "st_attrs" (attributes)

   * "st_obtype" (object type).

   Note: The exact meaning and resolution of the "st_atime",
     "st_mtime", and "st_ctime" attributes depend on the operating
     system and the file system. For example, on Windows systems using
     the FAT or FAT32 file systems, "st_mtime" has 2-second
     resolution, and "st_atime" has only 1-day resolution.  See your
     operating system documentation for details.

   For backward compatibility, the return value of "stat()" is also
   accessible as a tuple of at least 10 integers giving the most
   important (and portable) members of the "stat" structure, in the
   order "st_mode", "st_ino", "st_dev", "st_nlink", "st_uid",
   "st_gid", "st_size", "st_atime", "st_mtime", "st_ctime". More items
   may be added at the end by some implementations.

   The standard module "stat" defines functions and constants that are
   useful for extracting information from a "stat" structure. (On
   Windows, some items are filled with dummy values.)

   Example:

      >>> import os
      >>> statinfo = os.stat('somefile.txt')
      >>> statinfo
      (33188, 422511, 769, 1, 1032, 100, 926, 1105022698,1105022732, 1105022732)
      >>> statinfo.st_size
      926

   Availability: Unix, Windows.

   Changed in version 2.2: Added access to values as attributes of the
   returned object.

   Changed in version 2.5: Added "st_gen" and "st_birthtime".

os.stat_float_times([newvalue])

   Determine whether "stat_result" represents time stamps as float
   objects. If *newvalue* is "True", future calls to "stat()" return
   floats, if it is "False", future calls return ints. If *newvalue*
   is omitted, return the current setting.

   For compatibility with older Python versions, accessing
   "stat_result" as a tuple always returns integers.

   Changed in version 2.5: Python now returns float values by default.
   Applications which do not work correctly with floating point time
   stamps can use this function to restore the old behaviour.

   The resolution of the timestamps (that is the smallest possible
   fraction) depends on the system. Some systems only support second
   resolution; on these systems, the fraction will always be zero.

   It is recommended that this setting is only changed at program
   startup time in the *__main__* module; libraries should never
   change this setting. If an application uses a library that works
   incorrectly if floating point time stamps are processed, this
   application should turn the feature off until the library has been
   corrected.

os.statvfs(path)

   Perform a "statvfs()" system call on the given path.  The return
   value is an object whose attributes describe the filesystem on the
   given path, and correspond to the members of the "statvfs"
   structure, namely: "f_bsize", "f_frsize", "f_blocks", "f_bfree",
   "f_bavail", "f_files", "f_ffree", "f_favail", "f_flag",
   "f_namemax".

   For backward compatibility, the return value is also accessible as
   a tuple whose values correspond to the attributes, in the order
   given above. The standard module "statvfs" defines constants that
   are useful for extracting information from a "statvfs" structure
   when accessing it as a sequence; this remains useful when writing
   code that needs to work with versions of Python that don’t support
   accessing the fields as attributes.

   Availability: Unix.

   Changed in version 2.2: Added access to values as attributes of the
   returned object.

os.symlink(source, link_name)

   Create a symbolic link pointing to *source* named *link_name*.

   Availability: Unix.

os.tempnam([dir[, prefix]])

   Return a unique path name that is reasonable for creating a
   temporary file. This will be an absolute path that names a
   potential directory entry in the directory *dir* or a common
   location for temporary files if *dir* is omitted or "None".  If
   given and not "None", *prefix* is used to provide a short prefix to
   the filename.  Applications are responsible for properly creating
   and managing files created using paths returned by "tempnam()"; no
   automatic cleanup is provided. On Unix, the environment variable
   "TMPDIR" overrides *dir*, while on Windows "TMP" is used.  The
   specific behavior of this function depends on the C library
   implementation; some aspects are underspecified in system
   documentation.

   Warning: Use of "tempnam()" is vulnerable to symlink attacks;
     consider using "tmpfile()" (section File Object Creation)
     instead.

   Availability: Unix, Windows.

os.tmpnam()

   Return a unique path name that is reasonable for creating a
   temporary file. This will be an absolute path that names a
   potential directory entry in a common location for temporary files.
   Applications are responsible for properly creating and managing
   files created using paths returned by "tmpnam()"; no automatic
   cleanup is provided.

   Warning: Use of "tmpnam()" is vulnerable to symlink attacks;
     consider using "tmpfile()" (section File Object Creation)
     instead.

   Availability: Unix, Windows.  This function probably shouldn’t be
   used on Windows, though: Microsoft’s implementation of "tmpnam()"
   always creates a name in the root directory of the current drive,
   and that’s generally a poor location for a temp file (depending on
   privileges, you may not even be able to open a file using this
   name).

os.TMP_MAX

   The maximum number of unique names that "tmpnam()" will generate
   before reusing names.

os.unlink(path)

   Remove (delete) the file *path*.  This is the same function as
   "remove()"; the "unlink()" name is its traditional Unix name.

   Availability: Unix, Windows.

os.utime(path, times)

   Set the access and modified times of the file specified by *path*.
   If *times* is "None", then the file’s access and modified times are
   set to the current time. (The effect is similar to running the Unix
   program **touch** on the path.)  Otherwise, *times* must be a
   2-tuple of numbers, of the form "(atime, mtime)" which is used to
   set the access and modified times, respectively. Whether a
   directory can be given for *path* depends on whether the operating
   system implements directories as files (for example, Windows does
   not).  Note that the exact times you set here may not be returned
   by a subsequent "stat()" call, depending on the resolution with
   which your operating system records access and modification times;
   see "stat()".

   Changed in version 2.0: Added support for "None" for *times*.

   Availability: Unix, Windows.

os.walk(top, topdown=True, onerror=None, followlinks=False)

   Generate the file names in a directory tree by walking the tree
   either top-down or bottom-up. For each directory in the tree rooted
   at directory *top* (including *top* itself), it yields a 3-tuple
   "(dirpath, dirnames, filenames)".

   *dirpath* is a string, the path to the directory.  *dirnames* is a
   list of the names of the subdirectories in *dirpath* (excluding
   "'.'" and "'..'"). *filenames* is a list of the names of the non-
   directory files in *dirpath*. Note that the names in the lists
   contain no path components.  To get a full path (which begins with
   *top*) to a file or directory in *dirpath*, do
   "os.path.join(dirpath, name)".

   If optional argument *topdown* is "True" or not specified, the
   triple for a directory is generated before the triples for any of
   its subdirectories (directories are generated top-down).  If
   *topdown* is "False", the triple for a directory is generated after
   the triples for all of its subdirectories (directories are
   generated bottom-up). No matter the value of *topdown*, the list of
   subdirectories is retrieved before the tuples for the directory and
   its subdirectories are generated.

   When *topdown* is "True", the caller can modify the *dirnames* list
   in-place (perhaps using "del" or slice assignment), and "walk()"
   will only recurse into the subdirectories whose names remain in
   *dirnames*; this can be used to prune the search, impose a specific
   order of visiting, or even to inform "walk()" about directories the
   caller creates or renames before it resumes "walk()" again.
   Modifying *dirnames* when *topdown* is "False" has no effect on the
   behavior of the walk, because in bottom-up mode the directories in
   *dirnames* are generated before *dirpath* itself is generated.

   By default, errors from the "listdir()" call are ignored.  If
   optional argument *onerror* is specified, it should be a function;
   it will be called with one argument, an "OSError" instance.  It can
   report the error to continue with the walk, or raise the exception
   to abort the walk.  Note that the filename is available as the
   "filename" attribute of the exception object.

   By default, "walk()" will not walk down into symbolic links that
   resolve to directories. Set *followlinks* to "True" to visit
   directories pointed to by symlinks, on systems that support them.

   New in version 2.6: The *followlinks* parameter.

   Note: Be aware that setting *followlinks* to "True" can lead to
     infinite recursion if a link points to a parent directory of
     itself. "walk()" does not keep track of the directories it
     visited already.

   Note: If you pass a relative pathname, don’t change the current
     working directory between resumptions of "walk()".  "walk()"
     never changes the current directory, and assumes that its caller
     doesn’t either.

   This example displays the number of bytes taken by non-directory
   files in each directory under the starting directory, except that
   it doesn’t look under any CVS subdirectory:

      import os
      from os.path import join, getsize
      for root, dirs, files in os.walk('python/Lib/email'):
          print root, "consumes",
          print sum(getsize(join(root, name)) for name in files),
          print "bytes in", len(files), "non-directory files"
          if 'CVS' in dirs:
              dirs.remove('CVS')  # don't visit CVS directories

   In the next example, walking the tree bottom-up is essential:
   "rmdir()" doesn’t allow deleting a directory before the directory
   is empty:

      # Delete everything reachable from the directory named in "top",
      # assuming there are no symbolic links.
      # CAUTION:  This is dangerous!  For example, if top == '/', it
      # could delete all your disk files.
      import os
      for root, dirs, files in os.walk(top, topdown=False):
          for name in files:
              os.remove(os.path.join(root, name))
          for name in dirs:
              os.rmdir(os.path.join(root, name))

   New in version 2.3.


15.1.5. Process Management
==========================

These functions may be used to create and manage processes.

The various "exec*" functions take a list of arguments for the new
program loaded into the process.  In each case, the first of these
arguments is passed to the new program as its own name rather than as
an argument a user may have typed on a command line.  For the C
programmer, this is the "argv[0]" passed to a program’s "main()".  For
example, "os.execv('/bin/echo', ['foo', 'bar'])" will only print "bar"
on standard output; "foo" will seem to be ignored.

os.abort()

   Generate a "SIGABRT" signal to the current process.  On Unix, the
   default behavior is to produce a core dump; on Windows, the process
   immediately returns an exit code of "3".  Be aware that calling
   this function will not call the Python signal handler registered
   for "SIGABRT" with "signal.signal()".

   Availability: Unix, Windows.

os.execl(path, arg0, arg1, ...)
os.execle(path, arg0, arg1, ..., env)
os.execlp(file, arg0, arg1, ...)
os.execlpe(file, arg0, arg1, ..., env)
os.execv(path, args)
os.execve(path, args, env)
os.execvp(file, args)
os.execvpe(file, args, env)

   These functions all execute a new program, replacing the current
   process; they do not return.  On Unix, the new executable is loaded
   into the current process, and will have the same process id as the
   caller.  Errors will be reported as "OSError" exceptions.

   The current process is replaced immediately. Open file objects and
   descriptors are not flushed, so if there may be data buffered on
   these open files, you should flush them using "sys.stdout.flush()"
   or "os.fsync()" before calling an "exec*" function.

   The “l” and “v” variants of the "exec*" functions differ in how
   command-line arguments are passed.  The “l” variants are perhaps
   the easiest to work with if the number of parameters is fixed when
   the code is written; the individual parameters simply become
   additional parameters to the "execl*()" functions.  The “v”
   variants are good when the number of parameters is variable, with
   the arguments being passed in a list or tuple as the *args*
   parameter.  In either case, the arguments to the child process
   should start with the name of the command being run, but this is
   not enforced.

   The variants which include a “p” near the end ("execlp()",
   "execlpe()", "execvp()", and "execvpe()") will use the "PATH"
   environment variable to locate the program *file*.  When the
   environment is being replaced (using one of the "exec*e" variants,
   discussed in the next paragraph), the new environment is used as
   the source of the "PATH" variable. The other variants, "execl()",
   "execle()", "execv()", and "execve()", will not use the "PATH"
   variable to locate the executable; *path* must contain an
   appropriate absolute or relative path.

   For "execle()", "execlpe()", "execve()", and "execvpe()" (note that
   these all end in “e”), the *env* parameter must be a mapping which
   is used to define the environment variables for the new process
   (these are used instead of the current process’ environment); the
   functions "execl()", "execlp()", "execv()", and "execvp()" all
   cause the new process to inherit the environment of the current
   process.

   Availability: Unix, Windows.

os._exit(n)

   Exit the process with status *n*, without calling cleanup handlers,
   flushing stdio buffers, etc.

   Availability: Unix, Windows.

   Note: The standard way to exit is "sys.exit(n)".  "_exit()"
     should normally only be used in the child process after a
     "fork()".

The following exit codes are defined and can be used with "_exit()",
although they are not required.  These are typically used for system
programs written in Python, such as a mail server’s external command
delivery program.

Note: Some of these may not be available on all Unix platforms,
  since there is some variation.  These constants are defined where
  they are defined by the underlying platform.

os.EX_OK

   Exit code that means no error occurred.

   Availability: Unix.

   New in version 2.3.

os.EX_USAGE

   Exit code that means the command was used incorrectly, such as when
   the wrong number of arguments are given.

   Availability: Unix.

   New in version 2.3.

os.EX_DATAERR

   Exit code that means the input data was incorrect.

   Availability: Unix.

   New in version 2.3.

os.EX_NOINPUT

   Exit code that means an input file did not exist or was not
   readable.

   Availability: Unix.

   New in version 2.3.

os.EX_NOUSER

   Exit code that means a specified user did not exist.

   Availability: Unix.

   New in version 2.3.

os.EX_NOHOST

   Exit code that means a specified host did not exist.

   Availability: Unix.

   New in version 2.3.

os.EX_UNAVAILABLE

   Exit code that means that a required service is unavailable.

   Availability: Unix.

   New in version 2.3.

os.EX_SOFTWARE

   Exit code that means an internal software error was detected.

   Availability: Unix.

   New in version 2.3.

os.EX_OSERR

   Exit code that means an operating system error was detected, such
   as the inability to fork or create a pipe.

   Availability: Unix.

   New in version 2.3.

os.EX_OSFILE

   Exit code that means some system file did not exist, could not be
   opened, or had some other kind of error.

   Availability: Unix.

   New in version 2.3.

os.EX_CANTCREAT

   Exit code that means a user specified output file could not be
   created.

   Availability: Unix.

   New in version 2.3.

os.EX_IOERR

   Exit code that means that an error occurred while doing I/O on some
   file.

   Availability: Unix.

   New in version 2.3.

os.EX_TEMPFAIL

   Exit code that means a temporary failure occurred.  This indicates
   something that may not really be an error, such as a network
   connection that couldn’t be made during a retryable operation.

   Availability: Unix.

   New in version 2.3.

os.EX_PROTOCOL

   Exit code that means that a protocol exchange was illegal, invalid,
   or not understood.

   Availability: Unix.

   New in version 2.3.

os.EX_NOPERM

   Exit code that means that there were insufficient permissions to
   perform the operation (but not intended for file system problems).

   Availability: Unix.

   New in version 2.3.

os.EX_CONFIG

   Exit code that means that some kind of configuration error
   occurred.

   Availability: Unix.

   New in version 2.3.

os.EX_NOTFOUND

   Exit code that means something like “an entry was not found”.

   Availability: Unix.

   New in version 2.3.

os.fork()

   Fork a child process.  Return "0" in the child and the child’s
   process id in the parent.  If an error occurs "OSError" is raised.

   Note that some platforms including FreeBSD <= 6.3, Cygwin and OS/2
   EMX have known issues when using fork() from a thread.

   Warning: See "ssl" for applications that use the SSL module with
     fork().

   Availability: Unix.

os.forkpty()

   Fork a child process, using a new pseudo-terminal as the child’s
   controlling terminal. Return a pair of "(pid, fd)", where *pid* is
   "0" in the child, the new child’s process id in the parent, and
   *fd* is the file descriptor of the master end of the pseudo-
   terminal.  For a more portable approach, use the "pty" module.  If
   an error occurs "OSError" is raised.

   Availability: some flavors of Unix.

os.kill(pid, sig)

   Send signal *sig* to the process *pid*.  Constants for the specific
   signals available on the host platform are defined in the "signal"
   module.

   Windows: The "signal.CTRL_C_EVENT" and "signal.CTRL_BREAK_EVENT"
   signals are special signals which can only be sent to console
   processes which share a common console window, e.g., some
   subprocesses. Any other value for *sig* will cause the process to
   be unconditionally killed by the TerminateProcess API, and the exit
   code will be set to *sig*. The Windows version of "kill()"
   additionally takes process handles to be killed.

   New in version 2.7: Windows support

os.killpg(pgid, sig)

   Send the signal *sig* to the process group *pgid*.

   Availability: Unix.

   New in version 2.3.

os.nice(increment)

   Add *increment* to the process’s “niceness”.  Return the new
   niceness.

   Availability: Unix.

os.plock(op)

   Lock program segments into memory.  The value of *op* (defined in
   "<sys/lock.h>") determines which segments are locked.

   Availability: Unix.

os.popen(...)
os.popen2(...)
os.popen3(...)
os.popen4(...)

   Run child processes, returning opened pipes for communications.
   These functions are described in section File Object Creation.

os.spawnl(mode, path, ...)
os.spawnle(mode, path, ..., env)
os.spawnlp(mode, file, ...)
os.spawnlpe(mode, file, ..., env)
os.spawnv(mode, path, args)
os.spawnve(mode, path, args, env)
os.spawnvp(mode, file, args)
os.spawnvpe(mode, file, args, env)

   Execute the program *path* in a new process.

   (Note that the "subprocess" module provides more powerful
   facilities for spawning new processes and retrieving their results;
   using that module is preferable to using these functions.  Check
   especially the Replacing Older Functions with the subprocess Module
   section.)

   If *mode* is "P_NOWAIT", this function returns the process id of
   the new process; if *mode* is "P_WAIT", returns the process’s exit
   code if it exits normally, or "-signal", where *signal* is the
   signal that killed the process.  On Windows, the process id will
   actually be the process handle, so can be used with the "waitpid()"
   function.

   The “l” and “v” variants of the "spawn*" functions differ in how
   command-line arguments are passed.  The “l” variants are perhaps
   the easiest to work with if the number of parameters is fixed when
   the code is written; the individual parameters simply become
   additional parameters to the "spawnl*()" functions.  The “v”
   variants are good when the number of parameters is variable, with
   the arguments being passed in a list or tuple as the *args*
   parameter.  In either case, the arguments to the child process must
   start with the name of the command being run.

   The variants which include a second “p” near the end ("spawnlp()",
   "spawnlpe()", "spawnvp()", and "spawnvpe()") will use the "PATH"
   environment variable to locate the program *file*.  When the
   environment is being replaced (using one of the "spawn*e" variants,
   discussed in the next paragraph), the new environment is used as
   the source of the "PATH" variable.  The other variants, "spawnl()",
   "spawnle()", "spawnv()", and "spawnve()", will not use the "PATH"
   variable to locate the executable; *path* must contain an
   appropriate absolute or relative path.

   For "spawnle()", "spawnlpe()", "spawnve()", and "spawnvpe()" (note
   that these all end in “e”), the *env* parameter must be a mapping
   which is used to define the environment variables for the new
   process (they are used instead of the current process’
   environment); the functions "spawnl()", "spawnlp()", "spawnv()",
   and "spawnvp()" all cause the new process to inherit the
   environment of the current process.  Note that keys and values in
   the *env* dictionary must be strings; invalid keys or values will
   cause the function to fail, with a return value of "127".

   As an example, the following calls to "spawnlp()" and "spawnvpe()"
   are equivalent:

      import os
      os.spawnlp(os.P_WAIT, 'cp', 'cp', 'index.html', '/dev/null')

      L = ['cp', 'index.html', '/dev/null']
      os.spawnvpe(os.P_WAIT, 'cp', L, os.environ)

   Availability: Unix, Windows.  "spawnlp()", "spawnlpe()",
   "spawnvp()" and "spawnvpe()" are not available on Windows.
   "spawnle()" and "spawnve()" are not thread-safe on Windows; we
   advise you to use the "subprocess" module instead.

   New in version 1.6.

os.P_NOWAIT
os.P_NOWAITO

   Possible values for the *mode* parameter to the "spawn*" family of
   functions.  If either of these values is given, the "spawn*()"
   functions will return as soon as the new process has been created,
   with the process id as the return value.

   Availability: Unix, Windows.

   New in version 1.6.

os.P_WAIT

   Possible value for the *mode* parameter to the "spawn*" family of
   functions.  If this is given as *mode*, the "spawn*()" functions
   will not return until the new process has run to completion and
   will return the exit code of the process the run is successful, or
   "-signal" if a signal kills the process.

   Availability: Unix, Windows.

   New in version 1.6.

os.P_DETACH
os.P_OVERLAY

   Possible values for the *mode* parameter to the "spawn*" family of
   functions.  These are less portable than those listed above.
   "P_DETACH" is similar to "P_NOWAIT", but the new process is
   detached from the console of the calling process. If "P_OVERLAY" is
   used, the current process will be replaced; the "spawn*()" function
   will not return.

   Availability: Windows.

   New in version 1.6.

os.startfile(path[, operation])

   Start a file with its associated application.

   When *operation* is not specified or "'open'", this acts like
   double-clicking the file in Windows Explorer, or giving the file
   name as an argument to the **start** command from the interactive
   command shell: the file is opened with whatever application (if
   any) its extension is associated.

   When another *operation* is given, it must be a “command verb” that
   specifies what should be done with the file. Common verbs
   documented by Microsoft are "'print'" and  "'edit'" (to be used on
   files) as well as "'explore'" and "'find'" (to be used on
   directories).

   "startfile()" returns as soon as the associated application is
   launched. There is no option to wait for the application to close,
   and no way to retrieve the application’s exit status.  The *path*
   parameter is relative to the current directory.  If you want to use
   an absolute path, make sure the first character is not a slash
   ("'/'"); the underlying Win32 "ShellExecute()" function doesn’t
   work if it is.  Use the "os.path.normpath()" function to ensure
   that the path is properly encoded for Win32.

   Availability: Windows.

   New in version 2.0.

   New in version 2.5: The *operation* parameter.

os.system(command)

   Execute the command (a string) in a subshell.  This is implemented
   by calling the Standard C function "system()", and has the same
   limitations. Changes to "sys.stdin", etc. are not reflected in the
   environment of the executed command.

   On Unix, the return value is the exit status of the process encoded
   in the format specified for "wait()".  Note that POSIX does not
   specify the meaning of the return value of the C "system()"
   function, so the return value of the Python function is system-
   dependent.

   On Windows, the return value is that returned by the system shell
   after running *command*, given by the Windows environment variable
   "COMSPEC": on **command.com** systems (Windows 95, 98 and ME) this
   is always "0"; on **cmd.exe** systems (Windows NT, 2000 and XP)
   this is the exit status of the command run; on systems using a non-
   native shell, consult your shell documentation.

   The "subprocess" module provides more powerful facilities for
   spawning new processes and retrieving their results; using that
   module is preferable to using this function.  See the Replacing
   Older Functions with the subprocess Module section in the
   "subprocess" documentation for some helpful recipes.

   Availability: Unix, Windows.

os.times()

   Return a 5-tuple of floating point numbers indicating accumulated
   (processor or other) times, in seconds.  The items are: user time,
   system time, children’s user time, children’s system time, and
   elapsed real time since a fixed point in the past, in that order.
   See the Unix manual page *times(2)* or the corresponding Windows
   Platform API documentation. On Windows, only the first two items
   are filled, the others are zero.

   Availability: Unix, Windows

os.wait()

   Wait for completion of a child process, and return a tuple
   containing its pid and exit status indication: a 16-bit number,
   whose low byte is the signal number that killed the process, and
   whose high byte is the exit status (if the signal number is zero);
   the high bit of the low byte is set if a core file was produced.

   Availability: Unix.

os.waitpid(pid, options)

   The details of this function differ on Unix and Windows.

   On Unix: Wait for completion of a child process given by process id
   *pid*, and return a tuple containing its process id and exit status
   indication (encoded as for "wait()").  The semantics of the call
   are affected by the value of the integer *options*, which should be
   "0" for normal operation.

   If *pid* is greater than "0", "waitpid()" requests status
   information for that specific process.  If *pid* is "0", the
   request is for the status of any child in the process group of the
   current process.  If *pid* is "-1", the request pertains to any
   child of the current process.  If *pid* is less than "-1", status
   is requested for any process in the process group "-pid" (the
   absolute value of *pid*).

   An "OSError" is raised with the value of errno when the syscall
   returns -1.

   On Windows: Wait for completion of a process given by process
   handle *pid*, and return a tuple containing *pid*, and its exit
   status shifted left by 8 bits (shifting makes cross-platform use of
   the function easier). A *pid* less than or equal to "0" has no
   special meaning on Windows, and raises an exception. The value of
   integer *options* has no effect. *pid* can refer to any process
   whose id is known, not necessarily a child process. The "spawn*"
   functions called with "P_NOWAIT" return suitable process handles.

os.wait3(options)

   Similar to "waitpid()", except no process id argument is given and
   a 3-element tuple containing the child’s process id, exit status
   indication, and resource usage information is returned.  Refer to
   "resource"."getrusage()" for details on resource usage information.
   The option argument is the same as that provided to "waitpid()" and
   "wait4()".

   Availability: Unix.

   New in version 2.5.

os.wait4(pid, options)

   Similar to "waitpid()", except a 3-element tuple, containing the
   child’s process id, exit status indication, and resource usage
   information is returned. Refer to "resource"."getrusage()" for
   details on resource usage information.  The arguments to "wait4()"
   are the same as those provided to "waitpid()".

   Availability: Unix.

   New in version 2.5.

os.WNOHANG

   The option for "waitpid()" to return immediately if no child
   process status is available immediately. The function returns "(0,
   0)" in this case.

   Availability: Unix.

os.WCONTINUED

   This option causes child processes to be reported if they have been
   continued from a job control stop since their status was last
   reported.

   Availability: Some Unix systems.

   New in version 2.3.

os.WUNTRACED

   This option causes child processes to be reported if they have been
   stopped but their current state has not been reported since they
   were stopped.

   Availability: Unix.

   New in version 2.3.

The following functions take a process status code as returned by
"system()", "wait()", or "waitpid()" as a parameter.  They may be used
to determine the disposition of a process.

os.WCOREDUMP(status)

   Return "True" if a core dump was generated for the process,
   otherwise return "False".

   Availability: Unix.

   New in version 2.3.

os.WIFCONTINUED(status)

   Return "True" if the process has been continued from a job control
   stop, otherwise return "False".

   Availability: Unix.

   New in version 2.3.

os.WIFSTOPPED(status)

   Return "True" if the process has been stopped, otherwise return
   "False".

   Availability: Unix.

os.WIFSIGNALED(status)

   Return "True" if the process exited due to a signal, otherwise
   return "False".

   Availability: Unix.

os.WIFEXITED(status)

   Return "True" if the process exited using the *exit(2)* system
   call, otherwise return "False".

   Availability: Unix.

os.WEXITSTATUS(status)

   If "WIFEXITED(status)" is true, return the integer parameter to the
   *exit(2)* system call.  Otherwise, the return value is meaningless.

   Availability: Unix.

os.WSTOPSIG(status)

   Return the signal which caused the process to stop.

   Availability: Unix.

os.WTERMSIG(status)

   Return the signal which caused the process to exit.

   Availability: Unix.


15.1.6. Miscellaneous System Information
========================================

os.confstr(name)

   Return string-valued system configuration values. *name* specifies
   the configuration value to retrieve; it may be a string which is
   the name of a defined system value; these names are specified in a
   number of standards (POSIX, Unix 95, Unix 98, and others).  Some
   platforms define additional names as well. The names known to the
   host operating system are given as the keys of the "confstr_names"
   dictionary.  For configuration variables not included in that
   mapping, passing an integer for *name* is also accepted.

   If the configuration value specified by *name* isn’t defined,
   "None" is returned.

   If *name* is a string and is not known, "ValueError" is raised.  If
   a specific value for *name* is not supported by the host system,
   even if it is included in "confstr_names", an "OSError" is raised
   with "errno.EINVAL" for the error number.

   Availability: Unix

os.confstr_names

   Dictionary mapping names accepted by "confstr()" to the integer
   values defined for those names by the host operating system. This
   can be used to determine the set of names known to the system.

   Availability: Unix.

os.getloadavg()

   Return the number of processes in the system run queue averaged
   over the last 1, 5, and 15 minutes or raises "OSError" if the load
   average was unobtainable.

   Availability: Unix.

   New in version 2.3.

os.sysconf(name)

   Return integer-valued system configuration values. If the
   configuration value specified by *name* isn’t defined, "-1" is
   returned.  The comments regarding the *name* parameter for
   "confstr()" apply here as well; the dictionary that provides
   information on the known names is given by "sysconf_names".

   Availability: Unix.

os.sysconf_names

   Dictionary mapping names accepted by "sysconf()" to the integer
   values defined for those names by the host operating system. This
   can be used to determine the set of names known to the system.

   Availability: Unix.

The following data values are used to support path manipulation
operations.  These are defined for all platforms.

Higher-level operations on pathnames are defined in the "os.path"
module.

os.curdir

   The constant string used by the operating system to refer to the
   current directory. This is "'.'" for Windows and POSIX. Also
   available via "os.path".

os.pardir

   The constant string used by the operating system to refer to the
   parent directory. This is "'..'" for Windows and POSIX. Also
   available via "os.path".

os.sep

   The character used by the operating system to separate pathname
   components. This is "'/'" for POSIX and "'\\'" for Windows.  Note
   that knowing this is not sufficient to be able to parse or
   concatenate pathnames — use "os.path.split()" and "os.path.join()"
   — but it is occasionally useful. Also available via "os.path".

os.altsep

   An alternative character used by the operating system to separate
   pathname components, or "None" if only one separator character
   exists.  This is set to "'/'" on Windows systems where "sep" is a
   backslash. Also available via "os.path".

os.extsep

   The character which separates the base filename from the extension;
   for example, the "'.'" in "os.py". Also available via "os.path".

   New in version 2.2.

os.pathsep

   The character conventionally used by the operating system to
   separate search path components (as in "PATH"), such as "':'" for
   POSIX or "';'" for Windows. Also available via "os.path".

os.defpath

   The default search path used by "exec*p*" and "spawn*p*" if the
   environment doesn’t have a "'PATH'" key. Also available via
   "os.path".

os.linesep

   The string used to separate (or, rather, terminate) lines on the
   current platform.  This may be a single character, such as "'\n'"
   for POSIX, or multiple characters, for example, "'\r\n'" for
   Windows. Do not use *os.linesep* as a line terminator when writing
   files opened in text mode (the default); use a single "'\n'"
   instead, on all platforms.

os.devnull

   The file path of the null device. For example: "'/dev/null'" for
   POSIX, "'nul'" for Windows.  Also available via "os.path".

   New in version 2.4.


15.1.7. Miscellaneous Functions
===============================

os.urandom(n)

   Return a string of *n* random bytes suitable for cryptographic use.

   This function returns random bytes from an OS-specific randomness
   source.  The returned data should be unpredictable enough for
   cryptographic applications, though its exact quality depends on the
   OS implementation.  On a UNIX-like system this will query
   "/dev/urandom", and on Windows it will use "CryptGenRandom()".  If
   a randomness source is not found, "NotImplementedError" will be
   raised.

   For an easy-to-use interface to the random number generator
   provided by your platform, please see "random.SystemRandom".

   New in version 2.4.
