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Python 3.6.5 Documentation > "os" — Miscellaneous operating system interfaces

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

**Source code:** Lib/os.py

======================================================================

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'",
"'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.

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.

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 *path-like* *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.

Changed in version 3.6: Support added to accept objects
implementing the "os.PathLike" interface.

os.fsdecode(filename)

Decode the *path-like* *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.

Changed in version 3.6: Support added to accept objects
implementing the "os.PathLike" interface.

os.fspath(path)

Return the file system representation of the path.

If "str" or "bytes" is passed in, it is returned unchanged.
Otherwise "__fspath__()" is called and its value is returned as
long as it is a "str" or "bytes" object. In all other cases,
"TypeError" is raised.

New in version 3.6.

class os.PathLike

An *abstract base class* for objects representing a file system
path, e.g. "pathlib.PurePath".

New in version 3.6.

abstractmethod __fspath__()

Return the file system path representation of the object.

The method should only return a "str" or "bytes" object, with
the preference being for "str".

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.

"getenvb()" is only available if "supports_bytes_environ" is True.

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
"getpass.getuser()" since the latter checks the environment
variables "LOGNAME" or "USERNAME" to find out who the user is, and
falls back to "pwd.getpwuid(os.getuid())[0]" to get the login name
of the current real 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.

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 real 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.

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.

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.

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.

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*.

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

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.

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".

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)

Get the status of the file descriptor *fd*. Return a "stat_result"
object.

As of Python 3.3, this is equivalent to "os.stat(fd)".

See also: The "stat()" function.

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

Changed in version 3.3: Added the *dir_fd* parameter.

Changed in version 3.6: Accepts a *path-like object* for *src* and
*dst*.

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

Create a directory named *path* with numeric mode *mode*.

If the directory already exists, "FileExistsError" is raised.

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

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.

New in version 3.3: The *dir_fd* argument.

Changed in version 3.6: Accepts a *path-like object*.

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

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 bpo-21082.

Changed in version 3.6: Accepts a *path-like object*.

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.

Changed in version 3.6: Accepts a *path-like object*.

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

Create a filesystem node (file, device special file or named pipe)
named *path*. *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.

Availability: Unix.

New in version 3.3: The *dir_fd* argument.

Changed in version 3.6: Accepts a *path-like object*.

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.

This function can support specifying a file descriptor.

Availability: Unix.

Changed in version 3.6: Accepts a *path-like object*.

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 (directly or indirectly through a
"PathLike" interface), the result will also be a string object, and
the call may raise a UnicodeDecodeError. If the *path* is a bytes
object (direct or indirectly), 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.

Changed in version 3.6: Accepts a *path-like object*.

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 semantically identical to "unlink()".

New in version 3.3: The *dir_fd* argument.

Changed in version 3.6: Accepts a *path-like object*.

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.

Changed in version 3.6: Accepts a *path-like object*.

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()".

New in version 3.3: The *src_dir_fd* and *dst_dir_fd* arguments.

Changed in version 3.6: Accepts a *path-like object* for *src* and
*dst*.

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.

Changed in version 3.6: Accepts a *path-like object* for *old* and
*new*.

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.

New in version 3.3.

Changed in version 3.6: Accepts a *path-like object* for *src* and
*dst*.

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.

New in version 3.3: The *dir_fd* parameter.

Changed in version 3.6: Accepts a *path-like object*.

os.scandir(path='.')

Return an iterator of "os.DirEntry" objects corresponding to the
entries in the directory given by *path*. The entries are yielded
in arbitrary order, and the special entries "'.'" and "'..'" are
not included.

Using "scandir()" instead of "listdir()" can significantly increase
the performance of code that also needs file type or file attribute
information, because "os.DirEntry" objects expose this information
if the operating system provides it when scanning a directory. All
"os.DirEntry" methods may perform a system call, but "is_dir()" and
"is_file()" usually only require a system call for symbolic links;
"os.DirEntry.stat()" always requires a system call on Unix but only
requires one for symbolic links on Windows.

*path* may be a *path-like object*. If *path* is of type "bytes"
(directly or indirectly through the "PathLike" interface), the type
of the "name" and "path" attributes of each "os.DirEntry" will be
"bytes"; in all other circumstances, they will be of type "str".

The "scandir()" iterator supports the *context manager* protocol
and has the following method:

scandir.close()

Close the iterator and free acquired resources.

This is called automatically when the iterator is exhausted or
garbage collected, or when an error happens during iterating.
However it is advisable to call it explicitly or use the "with"
statement.

New in version 3.6.

The following example shows a simple use of "scandir()" to display
all the files (excluding directories) in the given *path* that
don’t start with "'.'". The "entry.is_file()" call will generally
not make an additional system call:

with os.scandir(path) as it:
for entry in it:
if not entry.name.startswith('.') and entry.is_file():
print(entry.name)

Note: On Unix-based systems, "scandir()" uses the system’s
opendir() and readdir() functions. On Windows, it uses the Win32
FindFirstFileW and FindNextFileW functions.

New in version 3.5.

New in version 3.6: Added support for the *context manager*
protocol and the "close()" method. If a "scandir()" iterator is
neither exhausted nor explicitly closed a "ResourceWarning" will be
emitted in its destructor.The function accepts a *path-like
object*.

class os.DirEntry

Object yielded by "scandir()" to expose the file path and other
file attributes of a directory entry.

"scandir()" will provide as much of this information as possible
without making additional system calls. When a "stat()" or
"lstat()" system call is made, the "os.DirEntry" object will cache
the result.

"os.DirEntry" instances are not intended to be stored in long-lived
data structures; if you know the file metadata has changed or if a
long time has elapsed since calling "scandir()", call
"os.stat(entry.path)" to fetch up-to-date information.

Because the "os.DirEntry" methods can make operating system calls,
they may also raise "OSError". If you need very fine-grained
control over errors, you can catch "OSError" when calling one of
the "os.DirEntry" methods and handle as appropriate.

To be directly usable as a *path-like object*, "os.DirEntry"
implements the "PathLike" interface.

Attributes and methods on a "os.DirEntry" instance are as follows:

name

The entry’s base filename, relative to the "scandir()" *path*
argument.

The "name" attribute will be "bytes" if the "scandir()" *path*
argument is of type "bytes" and "str" otherwise. Use
"fsdecode()" to decode byte filenames.

path

The entry’s full path name: equivalent to
"os.path.join(scandir_path, entry.name)" where *scandir_path* is
the "scandir()" *path* argument. The path is only absolute if
the "scandir()" *path* argument was absolute.

The "path" attribute will be "bytes" if the "scandir()" *path*
argument is of type "bytes" and "str" otherwise. Use
"fsdecode()" to decode byte filenames.

inode()

Return the inode number of the entry.

The result is cached on the "os.DirEntry" object. Use
"os.stat(entry.path, follow_symlinks=False).st_ino" to fetch up-
to-date information.

On the first, uncached call, a system call is required on
Windows but not on Unix.

is_dir(*, follow_symlinks=True)

Return "True" if this entry is a directory or a symbolic link
pointing to a directory; return "False" if the entry is or
points to any other kind of file, or if it doesn’t exist
anymore.

If *follow_symlinks* is "False", return "True" only if this
entry is a directory (without following symlinks); return
"False" if the entry is any other kind of file or if it doesn’t
exist anymore.

The result is cached on the "os.DirEntry" object, with a
separate cache for *follow_symlinks* "True" and "False". Call
"os.stat()" along with "stat.S_ISDIR()" to fetch up-to-date
information.

On the first, uncached call, no system call is required in most
cases. Specifically, for non-symlinks, neither Windows or Unix
require a system call, except on certain Unix file systems, such
as network file systems, that return "dirent.d_type ==
DT_UNKNOWN". If the entry is a symlink, a system call will be
required to follow the symlink unless *follow_symlinks* is
"False".

This method can raise "OSError", such as "PermissionError", but
"FileNotFoundError" is caught and not raised.

is_file(*, follow_symlinks=True)

Return "True" if this entry is a file or a symbolic link
pointing to a file; return "False" if the entry is or points to
a directory or other non-file entry, or if it doesn’t exist
anymore.

If *follow_symlinks* is "False", return "True" only if this
entry is a file (without following symlinks); return "False" if
the entry is a directory or other non-file entry, or if it
doesn’t exist anymore.

The result is cached on the "os.DirEntry" object. Caching,
system calls made, and exceptions raised are as per "is_dir()".

is_symlink()

Return "True" if this entry is a symbolic link (even if broken);
return "False" if the entry points to a directory or any kind of
file, or if it doesn’t exist anymore.

The result is cached on the "os.DirEntry" object. Call
"os.path.islink()" to fetch up-to-date information.

On the first, uncached call, no system call is required in most
cases. Specifically, neither Windows or Unix require a system
call, except on certain Unix file systems, such as network file
systems, that return "dirent.d_type == DT_UNKNOWN".

This method can raise "OSError", such as "PermissionError", but
"FileNotFoundError" is caught and not raised.

stat(*, follow_symlinks=True)

Return a "stat_result" object for this entry. This method
follows symbolic links by default; to stat a symbolic link add
the "follow_symlinks=False" argument.

On Unix, this method always requires a system call. On Windows,
it only requires a system call if *follow_symlinks* is "True"
and the entry is a symbolic link.

On Windows, the "st_ino", "st_dev" and "st_nlink" attributes of
the "stat_result" are always set to zero. Call "os.stat()" to
get these attributes.

The result is cached on the "os.DirEntry" object, with a
separate cache for *follow_symlinks* "True" and "False". Call
"os.stat()" to fetch up-to-date information.

Note that there is a nice correspondence between several attributes
and methods of "os.DirEntry" and of "pathlib.Path". In particular,
the "name" attribute has the same meaning, as do the "is_dir()",
"is_file()", "is_symlink()" and "stat()" methods.

New in version 3.5.

Changed in version 3.6: Added support for the "PathLike" interface.
Added support for "bytes" paths on Windows.

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

Get the status of a file or a file descriptor. Perform the
equivalent of a "stat()" system call on the given path. *path* may
be specified as either a string or bytes – directly or indirectly
through the "PathLike" interface – or as an open file descriptor.
Return a "stat_result" object.

This function normally follows symlinks; to stat a symlink add the
argument "follow_symlinks=False", or use "lstat()".

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

Example:

>>> import os
>>> statinfo = os.stat('somefile.txt')
>>> statinfo
os.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

See also: "fstat()" and "lstat()" functions.

New in version 3.3: Added the *dir_fd* and *follow_symlinks*
arguments, specifying a file descriptor instead of a path.

Changed in version 3.6: Accepts a *path-like object*.

class os.stat_result

Object whose attributes correspond roughly to the members of the
"stat" structure. It is used for the result of "os.stat()",
"os.fstat()" and "os.lstat()".

Attributes:

st_mode

File mode: file type and file mode bits (permissions).

st_ino

Inode number.

st_dev

Identifier of the device on which this file resides.

st_nlink

Number of hard links.

st_uid

User identifier of the file owner.

st_gid

Group identifier of the file owner.

st_size

Size of the file in bytes, if it is a regular file or a symbolic
link. The size of a symbolic link is the length of the pathname
it contains, without a terminating null byte.

Timestamps:

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:

* the time of most recent metadata change on Unix,

* 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:

* the time of most recent metadata change on Unix,

* the time of creation on Windows, expressed in nanoseconds as
an integer.

See also the "stat_float_times()" function.

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".

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

st_blocks

Number of 512-byte blocks allocated for file. This may be
smaller than "st_size"/512 when the file has holes.

st_blksize

“Preferred” blocksize for efficient file system I/O. Writing to
a file in smaller chunks may cause an inefficient read-modify-
rewrite.

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

Real size of the file.

st_creator

Creator of the file.

st_type

File type.

On Windows systems, the following attribute is also available:

st_file_attributes

Windows file attributes: "dwFileAttributes" member of the
"BY_HANDLE_FILE_INFORMATION" structure returned by
"GetFileInformationByHandle()". See the "FILE_ATTRIBUTE_*"
constants in the "stat" module.

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.)

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

New in version 3.3: Added the "st_atime_ns", "st_mtime_ns", and
"st_ctime_ns" members.

New in version 3.5: Added the "st_file_attributes" member on
Windows.

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.

Availability: Unix.

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

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

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.

Changed in version 3.6: Accepts a *path-like object*.

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_effective_ids",
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(src, dst, target_is_directory=False, *, dir_fd=None)

Create a symbolic link pointing to *src* named *dst*.

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.

Changed in version 3.6: Accepts a *path-like object* for *src* and
*dst*.

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, Windows.

New in version 3.3.

Changed in version 3.5: Added support for Windows

Changed in version 3.6: Accepts a *path-like object*.

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

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

New in version 3.3: The *dir_fd* parameter.

Changed in version 3.6: Accepts a *path-like object*.

os.utime(path, times=None, *[, ns], 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 specified, 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* is "None" and *ns* is unspecified, 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.

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

Changed in version 3.6: Accepts a *path-like object*.

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

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

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

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

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

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

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

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 (simple implementation of "shutil.rmtree()"),
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))

Changed in version 3.5: This function now calls "os.scandir()"
instead of "os.listdir()", making it faster by reducing the number
of calls to "os.stat()".

Changed in version 3.6: Accepts a *path-like object*.

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.

Changed in version 3.6: Accepts a *path-like object*.

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 (directly or indirectly
through the "PathLike" interface). If it is str, it is encoded with
the filesystem encoding.

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

Changed in version 3.6: Accepts a *path-like object* for *path* and
*attribute*.

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.

Changed in version 3.6: Accepts a *path-like object*.

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

Removes the extended filesystem attribute *attribute* from *path*.
*attribute* should be bytes or str (directly or indirectly through
the "PathLike" interface). If it is a string, it is encoded with
the filesystem encoding.

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

Changed in version 3.6: Accepts a *path-like object* for *path* and
*attribute*.

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
(directly or indirectly through the "PathLike" interface). 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.

Changed in version 3.6: Accepts a *path-like object* for *path* and
*attribute*.

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.

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()".

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()".

Changed in version 3.6: Accepts a *path-like object*.

os._exit(n)

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

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(cmd, mode='r', buffering=-1)

Open a pipe to or from command *cmd*. The return value is an open
file object connected to the pipe, which can be read or written
depending on whether *mode* is "'r'" (default) or "'w'". The
*buffering* argument has the same meaning as the corresponding
argument to the built-in "open()" function. The returned file
object reads or writes text strings rather than bytes.

The "close" method returns "None" if the subprocess exited
successfully, or the subprocess’s return code if there was an
error. On POSIX systems, if the return code is positive it
represents the return value of the process left-shifted by one
byte. If the return code is negative, the process was terminated
by the signal given by the negated value of the return code. (For
example, the return value might be "- signal.SIGKILL" if the
subprocess was killed.) On Windows systems, the return value
contains the signed integer return code from the child process.

This is implemented using "subprocess.Popen"; see that class’s
documentation for more powerful ways to manage and communicate with
subprocesses.

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.

Changed in version 3.6: Accepts a *path-like object*.

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.

To reduce interpreter startup overhead, the Win32 "ShellExecute()"
function is not resolved until this function is first called. If
the function cannot be resolved, "NotImplementedError" will be
raised.

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.

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 be 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.

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.

This number is not equivalent to the number of CPUs the current
process can use. The number of usable CPUs can be obtained with
"len(os.sched_getaffinity(0))"

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.

Random numbers
==============

os.getrandom(size, flags=0)

Get up to *size* random bytes. The function can return less bytes
than requested.

These bytes can be used to seed user-space random number generators
or for cryptographic purposes.

"getrandom()" relies on entropy gathered from device drivers and
other sources of environmental noise. Unnecessarily reading large
quantities of data will have a negative impact on other users of
the "/dev/random" and "/dev/urandom" devices.

The flags argument is a bit mask that can contain zero or more of
the following values ORed together: "os.GRND_RANDOM" and
"GRND_NONBLOCK".

See also the Linux getrandom() manual page.

Availability: Linux 3.17 and newer.

New in version 3.6.

os.urandom(size)

Return a string of *size* 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 Linux, if the "getrandom()" syscall is available, it is used in
blocking mode: block until the system urandom entropy pool is
initialized (128 bits of entropy are collected by the kernel). See
the **PEP 524** for the rationale. On Linux, the "getrandom()"
function can be used to get random bytes in non-blocking mode
(using the "GRND_NONBLOCK" flag) or to poll until the system
urandom entropy pool is initialized.

On a Unix-like system, random bytes are read from the
"/dev/urandom" device. If the "/dev/urandom" device is not
available or not readable, the "NotImplementedError" exception is
raised.

On Windows, it will use "CryptGenRandom()".

See also: The "secrets" module provides higher level functions.
For an easy-to-use interface to the random number generator
provided by your platform, please see "random.SystemRandom".

Changed in version 3.6.0: On Linux, "getrandom()" is now used in
blocking mode to increase the security.

Changed in version 3.5.2: On Linux, if the "getrandom()" syscall
blocks (the urandom entropy pool is not initialized yet), fall back
on reading "/dev/urandom".

Changed in version 3.5: On Linux 3.17 and newer, the "getrandom()"
syscall is now used when available. On OpenBSD 5.6 and newer, the
C "getentropy()" function is now used. These functions avoid the
usage of an internal file descriptor.

os.GRND_NONBLOCK

By default, when reading from "/dev/random", "getrandom()" blocks
if no random bytes are available, and when reading from
"/dev/urandom", it blocks if the entropy pool has not yet been
initialized.

If the "GRND_NONBLOCK" flag is set, then "getrandom()" does not
block in these cases, but instead immediately raises
"BlockingIOError".

New in version 3.6.

os.GRND_RANDOM

If this bit is set, then random bytes are drawn from the
"/dev/random" pool instead of the "/dev/urandom" pool.

New in version 3.6.

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