15.1.1. File Names, Command Line Arguments, and Environment Variables

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

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

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

15.1.2. Process Parameters

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

os.environ

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

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

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

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

Note

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

Note

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

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

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

os.environb

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

environb is only available if supports_bytes_environ is True.

New in version 3.2.

os.chdir(path)

os.fchdir(fd)

os.getcwd()

These functions are described in Files and Directories.

os.fsencode(filename)

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

fsdecode() is the reverse function.

New in version 3.2.

os.fsdecode(filename)

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

fsencode() is the reverse function.

New in version 3.2.

os.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.ctermid()

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

Availability: Unix.

os.getegid()

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

Availability: Unix.

os.geteuid()

Return the current process’s effective user id.

Availability: Unix.

os.getgid()

Return the real group id of the current process.

Availability: Unix.

os.getgroups()

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

Availability: Unix.

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.getlogin()

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

Availability: Unix, Windows.

os.getpgid(pid)

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

Availability: Unix.

os.getpgrp()

Return the id of the current process group.

Availability: Unix.

os.getpid()

Return the current process id.

Availability: Unix, Windows.

os.getppid()

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

Availability: Unix, Windows

Changed in version 3.2: Added support for Windows.

os.getresuid()

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

Availability: Unix.

New in version 3.2.

os.getresgid()

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

Availability: Unix.

New in version 3.2.

os.getuid()

Return the current process’s user id.

Availability: Unix.

os.getenv(key, default=None)

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

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

Availability: most flavors of Unix, Windows.

os.getenvb(key, default=None)

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

Availability: most flavors of Unix.

New in version 3.2.

os.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.

os.setpgrp()

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

Availability: Unix.

os.setpgid(pid, pgrp)

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

Availability: Unix.

os.setregid(rgid, egid)

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

Availability: Unix.

os.setresgid(rgid, egid, sgid)

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

Availability: Unix.

New in version 3.2.

os.setresuid(ruid, euid, suid)

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

Availability: Unix.

New in version 3.2.

os.setreuid(ruid, euid)

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

Availability: Unix.

os.getsid(pid)

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

Availability: Unix.

os.setsid()

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

Availability: Unix.

os.setuid(uid)

Set the current process’s user id.

Availability: Unix.

os.strerror(code)

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

Availability: Unix, Windows.

os.supports_bytes_environ

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

New in version 3.2.

os.umask(mask)

Set the current numeric umask and return the previous umask.

Availability: Unix, Windows.

os.uname()

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

Availability: recent flavors of Unix.

os.unsetenv(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.

15.1.3. File Object Creation

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

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

Return an open file object connected to the file descriptor fd. The mode and bufsize arguments have the same meaning as the corresponding arguments to the built-in open() function.

When specified, the mode argument must start with one of the letters 'r', 'w', or 'a', otherwise a ValueError is raised.

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

Availability: Unix, Windows.

15.1.4. File Descriptor Operations

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

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

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

os.close(fd)

Close file descriptor fd.

Availability: Unix, Windows.

Note

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

os.closerange(fd_low, fd_high)

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

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

Availability: Unix, Windows.

os.device_encoding(fd)

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

os.dup(fd)

Return a duplicate of file descriptor fd.

Availability: Unix, Windows.

os.dup2(fd, fd2)

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

Availability: Unix, Windows.

os.fchmod(fd, mode)

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

Availability: Unix.

os.fchown(fd, uid, gid)

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

Availability: Unix.

os.fdatasync(fd)

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

Availability: Unix.

Note

This function is not available on MacOS.

os.fpathconf(fd, name)

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

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

Availability: Unix.

os.fstat(fd)

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

Availability: Unix, Windows.

os.fstatvfs(fd)

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

Availability: Unix.

os.fsync(fd)

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

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

Availability: Unix, and Windows.

os.ftruncate(fd, length)

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

Availability: Unix.

os.isatty(fd)

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

Availability: Unix.

os.lseek(fd, pos, how)

Set the current position of file descriptor fd to position pos, modified by how: SEEK_SET or 0 to set the position relative to the beginning of the file; SEEK_CUR or 1 to set it relative to the current position; os.SEEK_END or 2 to set it relative to the end of the file.

Availability: Unix, Windows.

os.SEEK_SET

os.SEEK_CUR

os.SEEK_END

Parameters to the lseek() function. Their values are 0, 1, and 2, respectively. Availability: Windows, Unix.

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

Open the file file and set various flags according to flags and possibly its mode according to mode. The default mode is 0o777 (octal), and the current umask value is first masked out. Return the file descriptor for the newly opened file.

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

Availability: Unix, Windows.

Note

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

os.openpty()

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

Availability: some flavors of Unix.

os.pipe()

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

Availability: Unix, Windows.

os.read(fd, n)

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

Availability: Unix, Windows.

Note

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

os.tcgetpgrp(fd)

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

Availability: Unix.

os.tcsetpgrp(fd, pg)

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

Availability: Unix.

os.ttyname(fd)

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

Availability: Unix.

os.write(fd, str)

Write the bytestring in str to file descriptor fd. Return the number of bytes actually written.

Availability: Unix, Windows.

Note

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

15.1.5. Files and Directories

os.access(path, mode)

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

Availability: Unix, Windows.

Note

Using access() to check if a user is authorized to e.g. open a file before actually doing so using open() creates a security hole, because the user might exploit the short time interval between checking and opening the file to manipulate it.

Note

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

os.F_OK

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

os.R_OK

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

os.W_OK

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

os.X_OK

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

os.chdir(path)

Change the current working directory to path.

Availability: Unix, Windows.

os.fchdir(fd)

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

Availability: Unix.

os.getcwd()

Return a string representing the current working directory.

Availability: Unix, Windows.

os.getcwdb()

Return a bytestring representing the current working directory.

Availability: Unix, Windows.

os.chflags(path, flags)

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

• UF_NODUMP
• UF_IMMUTABLE
• UF_APPEND
• UF_OPAQUE
• UF_NOUNLINK
• SF_ARCHIVED
• SF_IMMUTABLE
• SF_APPEND
• SF_NOUNLINK
• SF_SNAPSHOT

Availability: Unix.

os.chroot(path)

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

os.chmod(path, mode)

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

• stat.S_ISUID
• stat.S_ISGID
• stat.S_ENFMT
• stat.S_ISVTX
• stat.S_IREAD
• stat.S_IWRITE
• stat.S_IEXEC
• stat.S_IRWXU
• stat.S_IRUSR
• stat.S_IWUSR
• stat.S_IXUSR
• stat.S_IRWXG
• stat.S_IRGRP
• stat.S_IWGRP
• stat.S_IXGRP
• stat.S_IRWXO
• stat.S_IROTH
• stat.S_IWOTH
• stat.S_IXOTH

Availability: Unix, Windows.

Note

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

os.chown(path, uid, gid)

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

Availability: Unix.

os.lchflags(path, flags)

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

Availability: Unix.

os.lchmod(path, mode)

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

Availability: Unix.

os.lchown(path, uid, gid)

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

Availability: Unix.

os.link(source, link_name)

Create a hard link pointing to source named link_name.

Availability: Unix, Windows.

Changed in version 3.2: Added Windows support.

os.listdir(path='.')

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

This function can be called with a bytes or string argument, and returns filenames of the same datatype.

Availability: Unix, Windows.

Changed in version 3.2: The path parameter became optional.

os.lstat(path)

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

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

os.mkfifo(path[, mode])

Create a FIFO (a named pipe) named path with numeric mode mode. The default mode is 0o666 (octal). The current umask value is first masked out from the mode.

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.

os.mknod(filename[, mode=0o600[, device]])

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

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.mkdir(path[, mode])

Create a directory named path with numeric mode mode. The default mode is 0o777 (octal). On some systems, mode is ignored. Where it is used, the current umask value is first masked out. If the directory already exists, OSError is raised.

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

Availability: Unix, Windows.

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

Recursive directory creation function. Like mkdir(), but makes all intermediate-level directories needed to contain the leaf directory. If the target directory with the same mode as specified already exists, raises an OSError exception if exist_ok is False, otherwise no exception is raised. If the directory cannot be created in other cases, raises an OSError exception. The default mode is 0o777 (octal). On some systems, mode is ignored. Where it is used, the current umask value is first masked out.

Note

makedirs() will become confused if the path elements to create include pardir.

This function handles UNC paths correctly.

New in version 3.2: The exist_ok parameter.

os.pathconf(path, name)

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

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

Availability: Unix.

os.pathconf_names

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

os.readlink(path)

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

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

Availability: Unix, Windows

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

os.remove(path)

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

Availability: Unix, Windows.

os.removedirs(path)

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

os.rename(src, dst)

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

Availability: Unix, Windows.

os.renames(old, new)

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

Note

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

os.rmdir(path)

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

Availability: Unix, Windows.

os.stat(path)

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

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

• st_mode - protection bits,
• st_ino - inode number,
• st_dev - device,
• st_nlink - number of hard links,
• st_uid - user id of owner,
• st_gid - group id of owner,
• st_size - size of file, in bytes,
• st_atime - time of most recent access,
• st_mtime - time of most recent content modification,
• st_ctime - platform dependent; time of most recent metadata change on Unix, or the time of creation on Windows)

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

• st_blocks - number of blocks allocated for file
• st_blksize - filesystem blocksize
• st_rdev - type of device if an inode device
• st_flags - user defined flags for file

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

• st_gen - file generation number
• st_birthtime - time of file creation

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

• st_rsize
• st_creator
• st_type

Note

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

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

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

Example:

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

Availability: Unix, Windows.

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.

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.

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

Availability: Unix.

os.symlink(source, link_name)

os.symlink(source, link_name, target_is_directory=False)

Create a symbolic link pointing to source named link_name.

On Windows, symlink version takes an additional optional parameter, target_is_directory, which defaults to False.

On Windows, a symlink represents a file or a directory, and does not morph to the target dynamically. For this reason, when creating a symlink on Windows, if the target is not already present, the symlink will default to being a file symlink. If target_is_directory is set to True, the symlink will be created as a directory symlink. This parameter is ignored if the target exists (and the symlink is created with the same type as the target).

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

Note

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.

os.unlink(path)

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

Availability: Unix, Windows.

os.utime(path, times)

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

Availability: Unix, Windows.

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

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

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

If optional argument topdown is True or not specified, the triple for a directory is generated before the triples for any of its subdirectories (directories are generated top-down). If topdown is False, the triple for a directory is generated after the triples for all of its subdirectories (directories are generated bottom-up).

When topdown is True, the caller can modify the dirnames list in-place (perhaps using del or slice assignment), and walk() will only recurse into the subdirectories whose names remain in dirnames; this can be used to prune the search, impose a specific order of visiting, or even to inform walk() about directories the caller creates or renames before it resumes walk() again. Modifying dirnames when topdown is False is ineffective, because in bottom-up mode the directories in dirnames are generated before dirpath itself is generated.

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

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

Note

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

Note

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

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

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

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

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

15.1.6. Process Management

These functions may be used to create and manage processes.

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

os.abort()

Generate a SIGABRT signal to the current process. On Unix, the default behavior is to produce a core dump; on Windows, the process immediately returns an exit code of 3. Be aware that programs which use signal.signal() to register a handler for SIGABRT will behave differently.

Availability: Unix, Windows.

os.execl(path, arg0, arg1, ...)

os.execle(path, arg0, arg1, ..., env)

os.execlp(file, arg0, arg1, ...)

os.execlpe(file, arg0, arg1, ..., env)

os.execv(path, args)

os.execve(path, args, env)

os.execvp(file, args)

os.execvpe(file, args, env)

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

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

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

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

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

Availability: Unix, Windows.

os._exit(n)

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

Availability: Unix, Windows.

Note

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

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

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, Cygwin and OS/2 EMX have known issues when using fork() from a thread.

Availability: Unix.

os.forkpty()

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

Availability: some flavors of Unix.

os.kill(pid, sig)

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

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

New in version 3.2: Windows support.

os.killpg(pgid, sig)

Send the signal sig to the process group pgid.

Availability: Unix.

os.nice(increment)

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

Availability: Unix.

os.plock(op)

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

Availability: Unix.

os.popen(...)

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

os.spawnl(mode, path, ...)

os.spawnle(mode, path, ..., env)

os.spawnlp(mode, file, ...)

os.spawnlpe(mode, file, ..., env)

os.spawnv(mode, path, args)

os.spawnve(mode, path, args, env)

os.spawnvp(mode, file, args)

os.spawnvpe(mode, file, args, env)

Execute the program path in a new process.

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

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

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

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

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

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

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

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

Availability: Unix, Windows. spawnlp(), spawnlpe(), spawnvp() and spawnvpe() are not available on Windows.

os.P_NOWAIT

os.P_NOWAITO

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

Availability: Unix, Windows.

os.P_WAIT

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

Availability: Unix, Windows.

os.P_DETACH

os.P_OVERLAY

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

Availability: Windows.

os.startfile(path[, operation])

Start a file with its associated application.

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

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

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

Availability: Windows.

os.system(command)

Execute the command (a string) in a subshell. This is implemented by calling the Standard C function system(), and has the same limitations. Changes to sys.stdin, etc. are not reflected in the environment of the executed command. If command generates any output, it will be sent to the interpreter standard output stream.

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

On Windows, the return value is that returned by the system shell after running command. The shell is given by the Windows environment variable COMSPEC: it is usually cmd.exe, which returns the exit status of the command run; on systems using a non-native shell, consult your shell documentation.

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

Availability: Unix, Windows.

os.times()

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

Availability: Unix, Windows

os.wait()

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

Availability: Unix.

os.waitpid(pid, options)

The details of this function differ on Unix and Windows.

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

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

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

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

os.wait3([options])

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

Availability: Unix.

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.

15.1.7. Miscellaneous System Information

os.confstr(name)

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

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

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

Availability: Unix

os.confstr_names

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

Availability: Unix.

os.getloadavg()

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

Availability: Unix.

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.

15.1.8. Miscellaneous Functions

os.urandom(n)

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

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