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# 16.1. `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.
- All functions accepting path or file names accept both bytes and string objects, and result in an object of the same type, if a path or file name is returned.
- 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'`, `'mac'`, `'ce'`, `'java'`.

  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.

## 16.1.1. File Names, Command Line Arguments, and Environment Variables

In Python, file names, command line arguments, and environment variables are represented using the string type. On some systems, decoding these strings to and from bytes is necessary before passing them to the operating system. Python uses the file system encoding to perform this conversion (see `sys.getfilesystemencoding()`).

Changed in version 3.1: On some systems, conversion using the file system encoding may fail. In this case, Python uses the `surrogateescape` encoding error handler, which means that undecodable bytes are replaced by a Unicode character U+DCxx on decoding, and these are again translated to the original byte on encoding.

The file system encoding must guarantee to successfully decode all bytes below 128. If the file system encoding fails to provide this guarantee, API functions may raise UnicodeErrors.

## 16.1.2. Process Parameters

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

`os.``ctermid`()

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

  Availability: Unix.

`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.

  On Unix, keys and values use `sys.getfilesystemencoding()` and `'surrogateescape'` error handler. Use `environb` if you would like to use a different encoding.

  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.

`os.``environb`

  Bytes version of `environ`: a mapping object representing the environment as byte strings. `environ` and `environb` are synchronized (modify `environb` updates `environ`, and vice versa).

  `environb` is only available if `supports_bytes_environ` is True.

  New in version 3.2.

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

  These functions are described in Files and Directories.

`os.``fsencode`(filename)

  Encode filename to the filesystem encoding with `'surrogateescape'` error handler, or `'strict'` on Windows; return `bytes` unchanged.

  `fsdecode()` is the reverse function.

  New in version 3.2.

`os.``fsdecode`(filename)

  Decode filename from the filesystem encoding with `'surrogateescape'` error handler, or `'strict'` on Windows; return `str` unchanged.

  `fsencode()` is the reverse function.

  New in version 3.2.

`os.``getenv`(key, default=None)

  Return the value of the environment variable key if it exists, or default if it doesn’t. key, default and the result are str.

  On Unix, keys and values are decoded with `sys.getfilesystemencoding()` and `'surrogateescape'` error handler. Use `os.getenvb()` if you would like to use a different encoding.

  Availability: most flavors of Unix, Windows.

`os.``getenvb`(key, default=None)

  Return the value of the environment variable key if it exists, or default if it doesn’t. key, default and the result are bytes.

  Availability: most flavors of Unix.

  New in version 3.2.

`os.``get_exec_path`(env=None)

  Returns the list of directories that will be searched for a named executable, similar to a shell, when launching a process. env, when specified, should be an environment variable dictionary to lookup the PATH in. By default, when env is None, `environ` is used.

  New in version 3.2.

`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.``getgrouplist`(user, group)

  Return list of group ids that user belongs to. If group is not in the list, it is included; typically, group is specified as the group ID field from the password record for user.

  Availability: Unix.

  New in version 3.3.

`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.``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 variables `LOGNAME` or `USERNAME` to find out who the user is, or `pwd.getpwuid(os.getuid())[0]` to get the login name of the currently effective user id.

  Availability: Unix, Windows.

`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.

`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. When the parent process has exited, on Unix the id returned is the one of the init process (1), on Windows it is still the same id, which may be already reused by another process.

  Availability: Unix, Windows.

  Changed in version 3.2: Added support for Windows.

`os.``getpriority`(which, who)

  Get program scheduling priority. The value which is one of `PRIO_PROCESS`, `PRIO_PGRP`, or `PRIO_USER`, and who is interpreted relative to which (a process identifier for `PRIO_PROCESS`, process group identifier for `PRIO_PGRP`, and a user ID for `PRIO_USER`). A zero value for who denotes (respectively) the calling process, the process group of the calling process, or the real user ID of the calling process.

  Availability: Unix.

  New in version 3.3.

`os.``PRIO_PROCESS`
`os.``PRIO_PGRP`
`os.``PRIO_USER`

  Parameters for the `getpriority()` and `setpriority()` functions.

  Availability: Unix.

  New in version 3.3.

`os.``getresuid`()

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

  Availability: Unix.

  New in version 3.2.

`os.``getresgid`()

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

  Availability: Unix.

  New in version 3.2.

`os.``getuid`()

  Return the current process’s user id.

  Availability: Unix.

`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 3.2.

`os.``putenv`(key, value)

  Set the environment variable named key 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.

  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.``setpriority`(which, who, priority)

  Set program scheduling priority. The value which is one of `PRIO_PROCESS`, `PRIO_PGRP`, or `PRIO_USER`, and who is interpreted relative to which (a process identifier for `PRIO_PROCESS`, process group identifier for `PRIO_PGRP`, and a user ID for `PRIO_USER`). A zero value for who denotes (respectively) the calling process, the process group of the calling process, or the real user ID of the calling process. priority is a value in the range -20 to 19. The default priority is 0; lower priorities cause more favorable scheduling.

  Availability: Unix

  New in version 3.3.

`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 3.2.

`os.``setresuid`(ruid, euid, suid)

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

  Availability: Unix.

  New in version 3.2.

`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.

`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.``supports_bytes_environ`

  `True` if the native OS type of the environment is bytes (eg. `False` on Windows).

  New in version 3.2.

`os.``umask`(mask)

  Set the current numeric umask and return the previous umask.

  Availability: Unix, Windows.

`os.``uname`()

  Returns information identifying the current operating system. The return value is an object with five attributes:

  - `sysname` - operating system name
  - `nodename` - name of machine on network (implementation-defined)
  - `release` - operating system release
  - `version` - operating system version
  - `machine` - hardware identifier

  For backwards compatibility, this object is also iterable, behaving like a five-tuple containing `sysname`, `nodename`, `release`, `version`, and `machine` in that order.

  Some systems truncate `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.

  Changed in version 3.3: Return type changed from a tuple to a tuple-like object with named attributes.

`os.``unsetenv`(key)

  Unset (delete) the environment variable named key. 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.

## 16.1.3. File Object Creation

This function creates new file objects. (See also `open()` for opening file descriptors.)

`os.``fdopen`(fd, *args, **kwargs)

  Return an open file object connected to the file descriptor fd. This is an alias of the `open()` built-in function and accepts the same arguments. The only difference is that the first argument of `fdopen()` must always be an integer.

## 16.1.4. 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 (but much faster than):

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

  Availability: Unix, Windows.

`os.``device_encoding`(fd)

  Return a string describing the encoding of the device associated with fd if it is connected to a terminal; else return `None`.

`os.``dup`(fd)

  Return a duplicate of file descriptor fd. The new file descriptor is non-inheritable.

  On Windows, when duplicating a standard stream (0: stdin, 1: stdout, 2: stderr), the new file descriptor is inheritable.

  Availability: Unix, Windows.

  Changed in version 3.4: The new file descriptor is now non-inheritable.

`os.``dup2`(fd, fd2, inheritable=True)

  Duplicate file descriptor fd to fd2, closing the latter first if necessary. The file descriptor fd2 is inheritable by default, or non-inheritable if inheritable is `False`.

  Availability: Unix, Windows.

  Changed in version 3.4: Add the optional inheritable parameter.

`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. As of Python 3.3, this is equivalent to `os.chmod(fd, mode)`.

  Availability: Unix.

`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. See `chown()`. As of Python 3.3, this is equivalent to `os.chown(fd, uid, gid)`.

  Availability: Unix.

`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.

  As of Python 3.3, this is equivalent to `os.pathconf(fd, name)`.

  Availability: Unix.

`os.``fstat`(fd)

  Return status for file descriptor fd, like `stat()`. As of Python 3.3, this is equivalent to `os.stat(fd)`.

  Availability: Unix, Windows.

`os.``fstatvfs`(fd)

  Return information about the filesystem containing the file associated with file descriptor fd, like `statvfs()`. As of Python 3.3, this is equivalent to `os.statvfs(fd)`.

  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 buffered 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, Windows.

`os.``ftruncate`(fd, length)

  Truncate the file corresponding to file descriptor fd, so that it is at most length bytes in size. As of Python 3.3, this is equivalent to `os.truncate(fd, length)`.

  Availability: Unix.

`os.``isatty`(fd)

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

`os.``lockf`(fd, cmd, len)

  Apply, test or remove a POSIX lock on an open file descriptor. fd is an open file descriptor. cmd specifies the command to use - one of `F_LOCK`, `F_TLOCK`, `F_ULOCK` or `F_TEST`. len specifies the section of the file to lock.

  Availability: Unix.

  New in version 3.3.

`os.``F_LOCK`
`os.``F_TLOCK`
`os.``F_ULOCK`
`os.``F_TEST`

  Flags that specify what action `lockf()` will take.

  Availability: Unix.

  New in version 3.3.

`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: Unix, Windows.

  New in version 3.3: Some operating systems could support additional values, like `os.SEEK_HOLE` or `os.SEEK_DATA`.

`os.``open`(file, flags, mode=0o777, *, dir_fd=None)

  Open the file file and set various flags according to flags and possibly its mode according to mode. When computing mode, the current umask value is first masked out. Return the file descriptor for the newly opened file. The new file descriptor is non-inheritable.

  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 the `os` module. In particular, on Windows adding `O_BINARY` is needed to open files in binary mode.

  This function can support paths relative to directory descriptors with the dir_fd parameter.

  Availability: Unix, Windows.

  Changed in version 3.4: The new file descriptor is now non-inheritable.

  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()`.

  New in version 3.3: The dir_fd argument.

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`

  These 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`
`os.``O_SHLOCK`
`os.``O_EXLOCK`
`os.``O_CLOEXEC`

  These constants are only available on Unix.

  Changed in version 3.3: Add `O_CLOEXEC` constant.

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

  These constants are only available on Windows.

`os.``O_ASYNC`
`os.``O_DIRECT`
`os.``O_DIRECTORY`
`os.``O_NOFOLLOW`
`os.``O_NOATIME`
`os.``O_PATH`
`os.``O_TMPFILE`

  These constants are GNU extensions and not present if they are not defined by the C library.

  Changed in version 3.4: Add `O_PATH` on systems that support it. Add `O_TMPFILE`, only available on Linux Kernel 3.11 or newer.

`os.``openpty`()

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

  Availability: some flavors of Unix.

  Changed in version 3.4: The new file descriptors are now non-inheritable.

`os.``pipe`()

  Create a pipe. Return a pair of file descriptors `(r, w)` usable for reading and writing, respectively. The new file descriptor is non-inheritable.

  Availability: Unix, Windows.

  Changed in version 3.4: The new file descriptors are now non-inheritable.

`os.``pipe2`(flags)

  Create a pipe with flags set atomically. flags can be constructed by ORing together one or more of these values: `O_NONBLOCK`, `O_CLOEXEC`. Return a pair of file descriptors `(r, w)` usable for reading and writing, respectively.

  Availability: some flavors of Unix.

  New in version 3.3.

`os.``posix_fallocate`(fd, offset, len)

  Ensures that enough disk space is allocated for the file specified by fd starting from offset and continuing for len bytes.

  Availability: Unix.

  New in version 3.3.

`os.``posix_fadvise`(fd, offset, len, advice)

  Announces an intention to access data in a specific pattern thus allowing the kernel to make optimizations. The advice applies to the region of the file specified by fd starting at offset and continuing for len bytes. advice is one of `POSIX_FADV_NORMAL`, `POSIX_FADV_SEQUENTIAL`, `POSIX_FADV_RANDOM`, `POSIX_FADV_NOREUSE`, `POSIX_FADV_WILLNEED` or `POSIX_FADV_DONTNEED`.

  Availability: Unix.

  New in version 3.3.

`os.``POSIX_FADV_NORMAL`
`os.``POSIX_FADV_SEQUENTIAL`
`os.``POSIX_FADV_RANDOM`
`os.``POSIX_FADV_NOREUSE`
`os.``POSIX_FADV_WILLNEED`
`os.``POSIX_FADV_DONTNEED`

  Flags that can be used in advice in `posix_fadvise()` that specify the access pattern that is likely to be used.

  Availability: Unix.

  New in version 3.3.

`os.``pread`(fd, buffersize, offset)

  Read from a file descriptor, fd, at a position of offset. It will read up to buffersize number of bytes. The file offset remains unchanged.

  Availability: Unix.

  New in version 3.3.

`os.``pwrite`(fd, string, offset)

  Write string to a file descriptor, fd, from offset, leaving the file offset unchanged.

  Availability: Unix.

  New in version 3.3.

`os.``read`(fd, n)

  Read at most n bytes from file descriptor fd. Return a bytestring containing the bytes read. If the end of the file referred to by fd has been reached, an empty bytes object 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.``sendfile`(out, in, offset, nbytes)
`os.``sendfile`(out, in, offset, nbytes, headers=None, trailers=None, flags=0)

  Copy nbytes bytes from file descriptor in to file descriptor out starting at offset. Return the number of bytes sent. When EOF is reached return 0.

  The first function notation is supported by all platforms that define `sendfile()`.

  On Linux, if offset is given as `None`, the bytes are read from the current position of in and the position of in is updated.

  The second case may be used on Mac OS X and FreeBSD where headers and trailers are arbitrary sequences of buffers that are written before and after the data from in is written. It returns the same as the first case.

  On Mac OS X and FreeBSD, a value of 0 for nbytes specifies to send until the end of in is reached.

  All platforms support sockets as out file descriptor, and some platforms allow other types (e.g. regular file, pipe) as well.

  Availability: Unix.

  New in version 3.3.

`os.``SF_NODISKIO`
`os.``SF_MNOWAIT`
`os.``SF_SYNC`

  Parameters to the `sendfile()` function, if the implementation supports them.

  Availability: Unix.

  New in version 3.3.

`os.``readv`(fd, buffers)

  Read from a file descriptor fd into a number of mutable bytes-like objects buffers. `readv()` will transfer data into each buffer until it is full and then move on to the next buffer in the sequence to hold the rest of the data. `readv()` returns the total number of bytes read (which may be less than the total capacity of all the objects).

  Availability: Unix.

  New in version 3.3.

`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 bytestring in 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.

`os.``writev`(fd, buffers)

  Write the contents of buffers to file descriptor fd. buffers must be a sequence of bytes-like objects. `writev()` writes the contents of each object to the file descriptor and returns the total number of bytes written.

  Availability: Unix.

  New in version 3.3.

### 16.1.4.1. Querying the size of a terminal

New in version 3.3.

`os.``get_terminal_size`(fd=STDOUT_FILENO)

  Return the size of the terminal window as `(columns, lines)`, tuple of type `terminal_size`.

  The optional argument `fd` (default `STDOUT_FILENO`, or standard output) specifies which file descriptor should be queried.

  If the file descriptor is not connected to a terminal, an `OSError` is raised.

  `shutil.get_terminal_size()` is the high-level function which should normally be used, `os.get_terminal_size` is the low-level implementation.

  Availability: Unix, Windows.

class `os.``terminal_size`

  A subclass of tuple, holding `(columns, lines)` of the terminal window size.

  `columns`

    Width of the terminal window in characters.

  `lines`

    Height of the terminal window in characters.

### 16.1.4.2. Inheritance of File Descriptors

New in version 3.4.

A file descriptor has an “inheritable” flag which indicates if the file descriptor can be inherited by child processes. Since Python 3.4, file descriptors created by Python are non-inheritable by default.

On UNIX, non-inheritable file descriptors are closed in child processes at the execution of a new program, other file descriptors are inherited.

On Windows, non-inheritable handles and file descriptors are closed in child processes, except for standard streams (file descriptors 0, 1 and 2: stdin, stdout and stderr), which are always inherited. Using `spawn*` functions, all inheritable handles and all inheritable file descriptors are inherited. Using the `subprocess` module, all file descriptors except standard streams are closed, and inheritable handles are only inherited if the close_fds parameter is `False`.

`os.``get_inheritable`(fd)

  Get the “inheritable” flag of the specified file descriptor (a boolean).

`os.``set_inheritable`(fd, inheritable)

  Set the “inheritable” flag of the specified file descriptor.

`os.``get_handle_inheritable`(handle)

  Get the “inheritable” flag of the specified handle (a boolean).

  Availability: Windows.

`os.``set_handle_inheritable`(handle, inheritable)

  Set the “inheritable” flag of the specified handle.

  Availability: Windows.

## 16.1.5. Files and Directories

On some Unix platforms, many of these functions support one or more of these features:

- specifying a file descriptor: For some functions, the path argument can be not only a string giving a path name, but also a file descriptor. The function will then operate on the file referred to by the descriptor. (For POSIX systems, Python will call the `f...` version of the function.)

  You can check whether or not path can be specified as a file descriptor on your platform using `os.supports_fd`. If it is unavailable, using it will raise a `NotImplementedError`.

  If the function also supports dir_fd or follow_symlinks arguments, it is an error to specify one of those when supplying path as a file descriptor.

- paths relative to directory descriptors: If dir_fd is not `None`, it should be a file descriptor referring to a directory, and the path to operate on should be relative; path will then be relative to that directory. If the path is absolute, dir_fd is ignored. (For POSIX systems, Python will call the `...at` or `f...at` version of the function.)

  You can check whether or not dir_fd is supported on your platform using `os.supports_dir_fd`. If it is unavailable, using it will raise a `NotImplementedError`.

- not following symlinks: If follow_symlinks is `False`, and the last element of the path to operate on is a symbolic link, the function will operate on the symbolic link itself instead of the file the link points to. (For POSIX systems, Python will call the `l...` version of the function.)

  You can check whether or not follow_symlinks is supported on your platform using `os.supports_follow_symlinks`. If it is unavailable, using it will raise a `NotImplementedError`.

`os.``access`(path, mode, *, dir_fd=None, effective_ids=False, follow_symlinks=True)

  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.

  This function can support specifying paths relative to directory descriptors and not following symlinks.

  If effective_ids is `True`, `access()` will perform its access checks using the effective uid/gid instead of the real uid/gid. effective_ids may not be supported on your platform; you can check whether or not it is available using `os.supports_effective_ids`. If it is unavailable, using it will raise a `NotImplementedError`.

  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 PermissionError:
          return "some default data"
      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.

  Changed in version 3.3: Added the dir_fd, effective_ids, and follow_symlinks parameters.

`os.``F_OK`
`os.``R_OK`
`os.``W_OK`
`os.``X_OK`

  Values to pass as the mode parameter of `access()` to test the existence, readability, writability and executability of path, respectively.

`os.``chdir`(path)

  Change the current working directory to path.

  This function can support specifying a file descriptor. The descriptor must refer to an opened directory, not an open file.

  Availability: Unix, Windows.

  New in version 3.3: Added support for specifying path as a file descriptor on some platforms.

`os.``chflags`(path, flags, *, follow_symlinks=True)

  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`

  This function can support not following symlinks.

  Availability: Unix.

  New in version 3.3: The follow_symlinks argument.

`os.``chmod`(path, mode, *, dir_fd=None, follow_symlinks=True)

  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`

  This function can support specifying a file descriptor, paths relative to directory descriptors and not following symlinks.

  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.

  New in version 3.3: Added support for specifying path as an open file descriptor, and the dir_fd and follow_symlinks arguments.

`os.``chown`(path, uid, gid, *, dir_fd=None, follow_symlinks=True)

  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.

  This function can support specifying a file descriptor, paths relative to directory descriptors and not following symlinks.

  See `shutil.chown()` for a higher-level function that accepts names in addition to numeric ids.

  Availability: Unix.

  New in version 3.3: Added support for specifying an open file descriptor for path, and the dir_fd and follow_symlinks arguments.

`os.``chroot`(path)

  Change the root directory of the current process to path.

  Availability: Unix.

`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. As of Python 3.3, this is equivalent to `os.chdir(fd)`.

  Availability: Unix.

`os.``getcwd`()

  Return a string representing the current working directory.

  Availability: Unix, Windows.

`os.``getcwdb`()

  Return a bytestring representing the current working directory.

  Availability: Unix, Windows.

`os.``lchflags`(path, flags)

  Set the flags of path to the numeric flags, like `chflags()`, but do not follow symbolic links. As of Python 3.3, this is equivalent to `os.chflags(path, flags, follow_symlinks=False)`.

  Availability: Unix.

`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. As of Python 3.3, this is equivalent to `os.chmod(path, mode, follow_symlinks=False)`.

  Availability: Unix.

`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. As of Python 3.3, this is equivalent to `os.chown(path, uid, gid, follow_symlinks=False)`.

  Availability: Unix.

`os.``link`(src, dst, *, src_dir_fd=None, dst_dir_fd=None, follow_symlinks=True)

  Create a hard link pointing to src named dst.

  This function can support specifying src_dir_fd and/or dst_dir_fd to supply paths relative to directory descriptors, and not following symlinks.

  Availability: Unix, Windows.

  Changed in version 3.2: Added Windows support.

  New in version 3.3: Added the src_dir_fd, dst_dir_fd, and follow_symlinks arguments.

`os.``listdir`(path='.')

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

  path may be either of type `str` or of type `bytes`. If path is of type `bytes`, the filenames returned will also be of type `bytes`; in all other circumstances, they will be of type `str`.

  This function can also support specifying a file descriptor; the file descriptor must refer to a directory.

  Note

  To encode `str` filenames to `bytes`, use `fsencode()`.

  Availability: Unix, Windows.

  Changed in version 3.2: The path parameter became optional.

  New in version 3.3: Added support for specifying an open file descriptor for path.

`os.``lstat`(path, *, dir_fd=None)

  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()`. As of Python 3.3, this is equivalent to `os.stat(path, dir_fd=dir_fd, follow_symlinks=False)`.

  This function can also support paths relative to directory descriptors.

  Changed in version 3.2: Added support for Windows 6.0 (Vista) symbolic links.

  Changed in version 3.3: Added the dir_fd parameter.

`os.``mkdir`(path, mode=0o777, *, dir_fd=None)

  Create a directory named path with numeric mode mode.

  On some systems, mode is ignored. Where it is used, the current umask value is first masked out. If the directory already exists, `OSError` is raised.

  This function can also support paths relative to directory descriptors.

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

  Availability: Unix, Windows.

  New in version 3.3: The dir_fd argument.

`os.``makedirs`(name, mode=0o777, exist_ok=False)

  Recursive directory creation function. Like `mkdir()`, but makes all intermediate-level directories needed to contain the leaf directory.

  The default mode is `0o777` (octal). On some systems, mode is ignored. Where it is used, the current umask value is first masked out.

  If exist_ok is `False` (the default), an `OSError` is raised if the target directory already exists.

  Note

  `makedirs()` will become confused if the path elements to create include `pardir` (eg. ”..” on UNIX systems).

  This function handles UNC paths correctly.

  New in version 3.2: The exist_ok parameter.

  Changed in version 3.4.1: Before Python 3.4.1, if exist_ok was `True` and the directory existed, `makedirs()` would still raise an error if mode did not match the mode of the existing directory. Since this behavior was impossible to implement safely, it was removed in Python 3.4.1. See issue 21082.

`os.``mkfifo`(path, mode=0o666, *, dir_fd=None)

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

  This function can also support paths relative to directory descriptors.

  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.

  Availability: Unix.

  New in version 3.3: The dir_fd argument.

`os.``mknod`(filename, mode=0o600, device=0, *, dir_fd=None)

  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.

  This function can also support paths relative to directory descriptors.

  New in version 3.3: The dir_fd argument.

`os.``major`(device)

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

`os.``minor`(device)

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

`os.``makedev`(major, minor)

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

`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.

  This function can support specifying a file descriptor.

  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, *, dir_fd=None)

  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)`.

  If the path is a string object, the result will also be a string object, and the call may raise an UnicodeDecodeError. If the path is a bytes object, the result will be a bytes object.

  This function can also support paths relative to directory descriptors.

  Availability: Unix, Windows

  Changed in version 3.2: Added support for Windows 6.0 (Vista) symbolic links.

  New in version 3.3: The dir_fd argument.

`os.``remove`(path, *, dir_fd=None)

  Remove (delete) the file path. If path is a directory, `OSError` is raised. Use `rmdir()` to remove directories.

  This function can support paths relative to directory descriptors.

  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.

  This function is identical to `unlink()`.

  Availability: Unix, Windows.

  New in version 3.3: The dir_fd argument.

`os.``removedirs`(name)

  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.

`os.``rename`(src, dst, *, src_dir_fd=None, dst_dir_fd=None)

  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.

  This function can support specifying src_dir_fd and/or dst_dir_fd to supply paths relative to directory descriptors.

  If you want cross-platform overwriting of the destination, use `replace()`.

  Availability: Unix, Windows.

  New in version 3.3: The src_dir_fd and dst_dir_fd arguments.

`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()`.

  Note

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

`os.``replace`(src, dst, *, src_dir_fd=None, dst_dir_fd=None)

  Rename the file or directory src to dst. If dst is a directory, `OSError` will be raised. If dst exists and is a file, it will be replaced silently if the user has permission. The operation may fail if src and dst are on different filesystems. If successful, the renaming will be an atomic operation (this is a POSIX requirement).

  This function can support specifying src_dir_fd and/or dst_dir_fd to supply paths relative to directory descriptors.

  Availability: Unix, Windows.

  New in version 3.3.

`os.``rmdir`(path, *, dir_fd=None)

  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.

  This function can support paths relative to directory descriptors.

  Availability: Unix, Windows.

  New in version 3.3: The dir_fd parameter.

`os.``stat`(path, *, dir_fd=None, follow_symlinks=True)

  Perform the equivalent of a `stat()` system call on the given path. path may be specified as either a string or as an open file descriptor. (This function normally follows symlinks; to stat a symlink add the argument `follow_symlinks=False`, or use `lstat()`.)

  The return value is an object whose attributes correspond roughly 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 expressed in seconds,
  - `st_mtime` - time of most recent content modification expressed in seconds,
  - `st_ctime` - platform dependent; time of most recent metadata change on Unix, or the time of creation on Windows, expressed in seconds
  - `st_atime_ns` - time of most recent access expressed in nanoseconds as an integer,
  - `st_mtime_ns` - time of most recent content modification expressed in nanoseconds as an integer,
  - `st_ctime_ns` - platform dependent; time of most recent metadata change on Unix, or the time of creation on Windows, expressed in nanoseconds as an integer

  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 Mac OS systems, the following attributes may also be available:

  - `st_rsize`
  - `st_creator`
  - `st_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. Similarly, although `st_atime_ns`, `st_mtime_ns`, and `st_ctime_ns` are always expressed in nanoseconds, many systems do not provide nanosecond precision. On systems that do provide nanosecond precision, the floating-point object used to store `st_atime`, `st_mtime`, and `st_ctime` cannot preserve all of it, and as such will be slightly inexact. If you need the exact timestamps you should always use `st_atime_ns`, `st_mtime_ns`, and `st_ctime_ns`.

  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.

  This function can support specifying a file descriptor and not following symlinks.

  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
      posix.stat_result(st_mode=33188, st_ino=7876932, st_dev=234881026,
      st_nlink=1, st_uid=501, st_gid=501, st_size=264, st_atime=1297230295,
      st_mtime=1297230027, st_ctime=1297230027)
      >>> statinfo.st_size
      264

  Availability: Unix, Windows.

  New in version 3.3: Added the dir_fd and follow_symlinks arguments, specifying a file descriptor instead of a path, and the `st_atime_ns`, `st_mtime_ns`, and `st_ctime_ns` members.

`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.

  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.

  Deprecated since version 3.3.

`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`.

  Two module-level constants are defined for the `f_flag` attribute’s bit-flags: if `ST_RDONLY` is set, the filesystem is mounted read-only, and if `ST_NOSUID` is set, the semantics of setuid/setgid bits are disabled or not supported.

  Additional module-level constants are defined for GNU/glibc based systems. These are `ST_NODEV` (disallow access to device special files), `ST_NOEXEC` (disallow program execution), `ST_SYNCHRONOUS` (writes are synced at once), `ST_MANDLOCK` (allow mandatory locks on an FS), `ST_WRITE` (write on file/directory/symlink), `ST_APPEND` (append-only file), `ST_IMMUTABLE` (immutable file), `ST_NOATIME` (do not update access times), `ST_NODIRATIME` (do not update directory access times), `ST_RELATIME` (update atime relative to mtime/ctime).

  This function can support specifying a file descriptor.

  Changed in version 3.2: The `ST_RDONLY` and `ST_NOSUID` constants were added.

  Changed in version 3.4: The `ST_NODEV`, `ST_NOEXEC`, `ST_SYNCHRONOUS`, `ST_MANDLOCK`, `ST_WRITE`, `ST_APPEND`, `ST_IMMUTABLE`, `ST_NOATIME`, `ST_NODIRATIME`, and `ST_RELATIME` constants were added.

  Availability: Unix.

  New in version 3.3: Added support for specifying an open file descriptor for path.

`os.``supports_dir_fd`

  A `Set` object indicating which functions in the `os` module permit use of their dir_fd parameter. Different platforms provide different functionality, and an option that might work on one might be unsupported on another. For consistency’s sakes, functions that support dir_fd always allow specifying the parameter, but will raise an exception if the functionality is not actually available.

  To check whether a particular function permits use of its dir_fd parameter, use the `in` operator on `supports_dir_fd`. As an example, this expression determines whether the dir_fd parameter of `os.stat()` is locally available:

      os.stat in os.supports_dir_fd

  Currently dir_fd parameters only work on Unix platforms; none of them work on Windows.

  New in version 3.3.

`os.``supports_effective_ids`

  A `Set` object indicating which functions in the `os` module permit use of the effective_ids parameter for `os.access()`. If the local platform supports it, the collection will contain `os.access()`, otherwise it will be empty.

  To check whether you can use the effective_ids parameter for `os.access()`, use the `in` operator on `supports_dir_fd`, like so:

      os.access in os.supports_effective_ids

  Currently effective_ids only works on Unix platforms; it does not work on Windows.

  New in version 3.3.

`os.``supports_fd`

  A `Set` object indicating which functions in the `os` module permit specifying their path parameter as an open file descriptor. Different platforms provide different functionality, and an option that might work on one might be unsupported on another. For consistency’s sakes, functions that support fd always allow specifying the parameter, but will raise an exception if the functionality is not actually available.

  To check whether a particular function permits specifying an open file descriptor for its path parameter, use the `in` operator on `supports_fd`. As an example, this expression determines whether `os.chdir()` accepts open file descriptors when called on your local platform:

      os.chdir in os.supports_fd

  New in version 3.3.

`os.``supports_follow_symlinks`

  A `Set` object indicating which functions in the `os` module permit use of their follow_symlinks parameter. Different platforms provide different functionality, and an option that might work on one might be unsupported on another. For consistency’s sakes, functions that support follow_symlinks always allow specifying the parameter, but will raise an exception if the functionality is not actually available.

  To check whether a particular function permits use of its follow_symlinks parameter, use the `in` operator on `supports_follow_symlinks`. As an example, this expression determines whether the follow_symlinks parameter of `os.stat()` is locally available:

      os.stat in os.supports_follow_symlinks

  New in version 3.3.

`os.``symlink`(source, link_name, target_is_directory=False, *, dir_fd=None)

  Create a symbolic link pointing to source named link_name.

  On Windows, a symlink represents either a file or a directory, and does not morph to the target dynamically. If the target is present, the type of the symlink will be created to match. Otherwise, the symlink will be created as a directory if target_is_directory is `True` or a file symlink (the default) otherwise. On non-Window platforms, target_is_directory is ignored.

  Symbolic link support was introduced in Windows 6.0 (Vista). `symlink()` will raise a `NotImplementedError` on Windows versions earlier than 6.0.

  This function can support paths relative to directory descriptors.

  Note

  On Windows, the SeCreateSymbolicLinkPrivilege is required in order to successfully create symlinks. This privilege is not typically granted to regular users but is available to accounts which can escalate privileges to the administrator level. Either obtaining the privilege or running your application as an administrator are ways to successfully create symlinks.

  `OSError` is raised when the function is called by an unprivileged user.

  Availability: Unix, Windows.

  Changed in version 3.2: Added support for Windows 6.0 (Vista) symbolic links.

  New in version 3.3: Added the dir_fd argument, and now allow target_is_directory on non-Windows platforms.

`os.``sync`()

  Force write of everything to disk.

  Availability: Unix.

  New in version 3.3.

`os.``truncate`(path, length)

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

  This function can support specifying a file descriptor.

  Availability: Unix.

  New in version 3.3.

`os.``unlink`(path, *, dir_fd=None)

  Remove (delete) the file path. This function is identical to `remove()`; the `unlink` name is its traditional Unix name. Please see the documentation for `remove()` for further information.

  Availability: Unix, Windows.

  New in version 3.3: The dir_fd parameter.

`os.``utime`(path, times=None, *, ns=None, dir_fd=None, follow_symlinks=True)

  Set the access and modified times of the file specified by path.

  `utime()` takes two optional parameters, times and ns. These specify the times set on path and are used as follows:

  - If ns is not `None`, it must be a 2-tuple of the form `(atime_ns, mtime_ns)` where each member is an int expressing nanoseconds.
  - If times is not `None`, it must be a 2-tuple of the form `(atime, mtime)` where each member is an int or float expressing seconds.
  - If times and ns are both `None`, this is equivalent to specifying `ns=(atime_ns, mtime_ns)` where both times are the current time.

  It is an error to specify tuples for both times and ns.

  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()`. The best way to preserve exact times is to use the st_atime_ns and st_mtime_ns fields from the `os.stat()` result object with the ns parameter to utime.

  This function can support specifying a file descriptor, paths relative to directory descriptors and not following symlinks.

  Availability: Unix, Windows.

  New in version 3.3: Added support for specifying an open file descriptor for path, and the dir_fd, follow_symlinks, and ns parameters.

`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).

  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` is ineffective, 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.

  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", end=" ")
          print(sum(getsize(join(root, name)) for name in files), end=" ")
          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))

`os.``fwalk`(top='.', topdown=True, onerror=None, *, follow_symlinks=False, dir_fd=None)

  This behaves exactly like `walk()`, except that it yields a 4-tuple `(dirpath, dirnames, filenames, dirfd)`, and it supports `dir_fd`.

  dirpath, dirnames and filenames are identical to `walk()` output, and dirfd is a file descriptor referring to the directory dirpath.

  This function always supports paths relative to directory descriptors and not following symlinks. Note however that, unlike other functions, the `fwalk()` default value for follow_symlinks is `False`.

  Note

  Since `fwalk()` yields file descriptors, those are only valid until the next iteration step, so you should duplicate them (e.g. with `dup()`) if you want to keep them longer.

  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
      for root, dirs, files, rootfd in os.fwalk('python/Lib/email'):
          print(root, "consumes", end="")
          print(sum([os.stat(name, dir_fd=rootfd).st_size for name in files]),
                end="")
          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, rootfd in os.fwalk(top, topdown=False):
          for name in files:
              os.unlink(name, dir_fd=rootfd)
          for name in dirs:
              os.rmdir(name, dir_fd=rootfd)

  Availability: Unix.

  New in version 3.3.

### 16.1.5.1. Linux extended attributes

New in version 3.3.

These functions are all available on Linux only.

`os.``getxattr`(path, attribute, *, follow_symlinks=True)

  Return the value of the extended filesystem attribute attribute for path. attribute can be bytes or str. If it is str, it is encoded with the filesystem encoding.

  This function can support specifying a file descriptor and not following symlinks.

`os.``listxattr`(path=None, *, follow_symlinks=True)

  Return a list of the extended filesystem attributes on path. The attributes in the list are represented as strings decoded with the filesystem encoding. If path is `None`, `listxattr()` will examine the current directory.

  This function can support specifying a file descriptor and not following symlinks.

`os.``removexattr`(path, attribute, *, follow_symlinks=True)

  Removes the extended filesystem attribute attribute from path. attribute should be bytes or str. If it is a string, it is encoded with the filesystem encoding.

  This function can support specifying a file descriptor and not following symlinks.

`os.``setxattr`(path, attribute, value, flags=0, *, follow_symlinks=True)

  Set the extended filesystem attribute attribute on path to value. attribute must be a bytes or str with no embedded NULs. If it is a str, it is encoded with the filesystem encoding. flags may be `XATTR_REPLACE` or `XATTR_CREATE`. If `XATTR_REPLACE` is given and the attribute does not exist, `EEXISTS` will be raised. If `XATTR_CREATE` is given and the attribute already exists, the attribute will not be created and `ENODATA` will be raised.

  This function can support specifying a file descriptor and not following symlinks.

  Note

  A bug in Linux kernel versions less than 2.6.39 caused the flags argument to be ignored on some filesystems.

`os.``XATTR_SIZE_MAX`

  The maximum size the value of an extended attribute can be. Currently, this is 64 KiB on Linux.

`os.``XATTR_CREATE`

  This is a possible value for the flags argument in `setxattr()`. It indicates the operation must create an attribute.

`os.``XATTR_REPLACE`

  This is a possible value for the flags argument in `setxattr()`. It indicates the operation must replace an existing attribute.

## 16.1.6. 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.

  For `execve()` on some platforms, path may also be specified as an open file descriptor. This functionality may not be supported on your platform; you can check whether or not it is available using `os.supports_fd`. If it is unavailable, using it will raise a `NotImplementedError`.

  Availability: Unix, Windows.

  New in version 3.3: Added support for specifying an open file descriptor for path for `execve()`.

`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.

`os.``EX_USAGE`

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

  Availability: Unix.

`os.``EX_DATAERR`

  Exit code that means the input data was incorrect.

  Availability: Unix.

`os.``EX_NOINPUT`

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

  Availability: Unix.

`os.``EX_NOUSER`

  Exit code that means a specified user did not exist.

  Availability: Unix.

`os.``EX_NOHOST`

  Exit code that means a specified host did not exist.

  Availability: Unix.

`os.``EX_UNAVAILABLE`

  Exit code that means that a required service is unavailable.

  Availability: Unix.

`os.``EX_SOFTWARE`

  Exit code that means an internal software error was detected.

  Availability: Unix.

`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.

`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.

`os.``EX_CANTCREAT`

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

  Availability: Unix.

`os.``EX_IOERR`

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

  Availability: Unix.

`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.

`os.``EX_PROTOCOL`

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

  Availability: Unix.

`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.

`os.``EX_CONFIG`

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

  Availability: Unix.

`os.``EX_NOTFOUND`

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

  Availability: Unix.

`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 and Cygwin 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.

  See also `signal.pthread_kill()`.

  New in version 3.2: Windows support.

`os.``killpg`(pgid, sig)

  Send the signal sig to the process group pgid.

  Availability: Unix.

`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`(...)

  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.

`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.

`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.

`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.

`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.

`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. If command generates any output, it will be sent to the interpreter standard output stream.

  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. The shell is given by the Windows environment variable `COMSPEC`: it is usually cmd.exe, which returns 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`()

  Returns the current global process times. The return value is an object with five attributes:

  - `user` - user time
  - `system` - system time
  - `children_user` - user time of all child processes
  - `children_system` - system time of all child processes
  - `elapsed` - elapsed real time since a fixed point in the past

  For backwards compatibility, this object also behaves like a five-tuple containing `user`, `system`, `children_user`, `children_system`, and `elapsed` in that order.

  See the Unix manual page times(2) or the corresponding Windows Platform API documentation. On Windows, only `user` and `system` are known; the other attributes are zero.

  Availability: Unix, Windows.

  Changed in version 3.3: Return type changed from a tuple to a tuple-like object with named attributes.

`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.``waitid`(idtype, id, options)

  Wait for the completion of one or more child processes. idtype can be `P_PID`, `P_PGID` or `P_ALL`. id specifies the pid to wait on. options is constructed from the ORing of one or more of `WEXITED`, `WSTOPPED` or `WCONTINUED` and additionally may be ORed with `WNOHANG` or `WNOWAIT`. The return value is an object representing the data contained in the `siginfo_t` structure, namely: `si_pid`, `si_uid`, `si_signo`, `si_status`, `si_code` or `None` if `WNOHANG` is specified and there are no children in a waitable state.

  Availability: Unix.

  New in version 3.3.

`os.``P_PID`
`os.``P_PGID`
`os.``P_ALL`

  These are the possible values for idtype in `waitid()`. They affect how id is interpreted.

  Availability: Unix.

  New in version 3.3.

`os.``WEXITED`
`os.``WSTOPPED`
`os.``WNOWAIT`

  Flags that can be used in options in `waitid()` that specify what child signal to wait for.

  Availability: Unix.

  New in version 3.3.

`os.``CLD_EXITED`
`os.``CLD_DUMPED`
`os.``CLD_TRAPPED`
`os.``CLD_CONTINUED`

  These are the possible values for `si_code` in the result returned by `waitid()`.

  Availability: Unix.

  New in version 3.3.

`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.

`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.

`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.

`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.

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.

`os.``WIFCONTINUED`(status)

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

  Availability: Unix.

`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.

## 16.1.7. Interface to the scheduler

These functions control how a process is allocated CPU time by the operating system. They are only available on some Unix platforms. For more detailed information, consult your Unix manpages.

New in version 3.3.

The following scheduling policies are exposed if they are supported by the operating system.

`os.``SCHED_OTHER`

  The default scheduling policy.

`os.``SCHED_BATCH`

  Scheduling policy for CPU-intensive processes that tries to preserve interactivity on the rest of the computer.

`os.``SCHED_IDLE`

  Scheduling policy for extremely low priority background tasks.

`os.``SCHED_SPORADIC`

  Scheduling policy for sporadic server programs.

`os.``SCHED_FIFO`

  A First In First Out scheduling policy.

`os.``SCHED_RR`

  A round-robin scheduling policy.

`os.``SCHED_RESET_ON_FORK`

  This flag can OR’ed with any other scheduling policy. When a process with this flag set forks, its child’s scheduling policy and priority are reset to the default.

class `os.``sched_param`(sched_priority)

  This class represents tunable scheduling parameters used in `sched_setparam()`, `sched_setscheduler()`, and `sched_getparam()`. It is immutable.

  At the moment, there is only one possible parameter:

  `sched_priority`

    The scheduling priority for a scheduling policy.

`os.``sched_get_priority_min`(policy)

  Get the minimum priority value for policy. policy is one of the scheduling policy constants above.

`os.``sched_get_priority_max`(policy)

  Get the maximum priority value for policy. policy is one of the scheduling policy constants above.

`os.``sched_setscheduler`(pid, policy, param)

  Set the scheduling policy for the process with PID pid. A pid of 0 means the calling process. policy is one of the scheduling policy constants above. param is a `sched_param` instance.

`os.``sched_getscheduler`(pid)

  Return the scheduling policy for the process with PID pid. A pid of 0 means the calling process. The result is one of the scheduling policy constants above.

`os.``sched_setparam`(pid, param)

  Set a scheduling parameters for the process with PID pid. A pid of 0 means the calling process. param is a `sched_param` instance.

`os.``sched_getparam`(pid)

  Return the scheduling parameters as a `sched_param` instance for the process with PID pid. A pid of 0 means the calling process.

`os.``sched_rr_get_interval`(pid)

  Return the round-robin quantum in seconds for the process with PID pid. A pid of 0 means the calling process.

`os.``sched_yield`()

  Voluntarily relinquish the CPU.

`os.``sched_setaffinity`(pid, mask)

  Restrict the process with PID pid (or the current process if zero) to a set of CPUs. mask is an iterable of integers representing the set of CPUs to which the process should be restricted.

`os.``sched_getaffinity`(pid)

  Return the set of CPUs the process with PID pid (or the current process if zero) is restricted to.

## 16.1.8. 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.``cpu_count`()

  Return the number of CPUs in the system. Returns None if undetermined.

  New in version 3.4.

`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.

`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`.

`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`.

`os.``RTLD_LAZY`
`os.``RTLD_NOW`
`os.``RTLD_GLOBAL`
`os.``RTLD_LOCAL`
`os.``RTLD_NODELETE`
`os.``RTLD_NOLOAD`
`os.``RTLD_DEEPBIND`

  Flags for use with the `setdlopenflags()` and `getdlopenflags()` functions. See the Unix manual page dlopen(3) for what the different flags mean.

  New in version 3.3.

## 16.1.9. 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`.

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