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:
Note
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.
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.
These functions and data items provide information and operate on the current process and user.
A mapping object representing the string environment. For example, environ['HOME'] is the pathname of your home directory (on some platforms), and is equivalent to getenv("HOME") in C.
This mapping is captured the first time the os module is imported, typically during Python startup as part of processing site.py. Changes to the environment made after this time are not reflected in os.environ, except for changes made by modifying os.environ directly.
If the platform supports the putenv() function, this mapping may be used to modify the environment as well as query the environment. putenv() will be called automatically when the mapping is modified.
Note
Calling putenv() directly does not change os.environ, so it’s better to modify os.environ.
Note
On some platforms, including FreeBSD and Mac OS X, setting environ may cause memory leaks. Refer to the system documentation for putenv().
If putenv() is not provided, a modified copy of this mapping may be passed to the appropriate process-creation functions to cause child processes to use a modified environment.
If the platform supports the unsetenv() function, you can delete items in this mapping to unset environment variables. unsetenv() will be called automatically when an item is deleted from os.environ, and when one of the pop() or clear() methods is called.
Return the current process’s effective user id. Availability: Unix.
Return the real group id of the current process. Availability: Unix.
Return the id of the current process group. Availability: Unix.
Return the current process id. Availability: Unix, Windows.
Return the parent’s process id. Availability: Unix.
Return the current process’s user id. Availability: Unix.
Set the environment variable named varname to the string value. Such changes to the environment affect subprocesses started with os.system(), popen() or fork() and execv(). Availability: most flavors of Unix, Windows.
Note
On some platforms, including FreeBSD and Mac OS X, setting environ may cause memory leaks. Refer to the system documentation for putenv.
When putenv() is supported, assignments to items in os.environ are automatically translated into corresponding calls to putenv(); however, calls to putenv() don’t update os.environ, so it is actually preferable to assign to items of os.environ.
Set the current process’s user id. Availability: Unix.
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.
Unset (delete) the environment variable named varname. Such changes to the environment affect subprocesses started with os.system(), popen() or fork() and execv(). Availability: most flavors of Unix, Windows.
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.
These functions create new file objects. (See also open().)
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. Availability: Unix, Windows.
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).
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.
Close file descriptor fd. Availability: Unix, Windows.
Close all file descriptors from fd_low (inclusive) to fd_high (exclusive), ignoring errors. Availability: Unix, Windows. Equivalent to:
for fd in range(fd_low, fd_high):
try:
os.close(fd)
except OSError:
pass
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.
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. Availability: Unix.
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.
Force write of file with filedescriptor fd to disk. On Unix, this calls the native fsync() function; on Windows, the MS _commit() function.
If you’re starting with a Python file object f, first do f.flush(), and then do os.fsync(f.fileno()), to ensure that all internal buffers associated with f are written to disk. Availability: Unix, Windows.
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. Availability: Unix, Windows.
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 below).
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.
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.
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.
The following constants are options for the flags parameter to the open() function. They can be combined using the bitwise OR operator |. Some of them are not available on all platforms. For descriptions of their availability and use, consult the open(2) manual page on Unix or the MSDN on Windows.
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.
Change the current working directory to path. Availability: Unix, Windows.
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):
Availability: Unix.
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:
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.
Return a list containing the names of the entries in the directory given by path. The list is in arbitrary order. It does not include the special entries '.' and '..' even if they are present in the directory. Availability: Unix, Windows.
This function can be called with a bytes or string argument, and returns filenames of the same datatype.
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. Availability: Unix.
FIFOs are pipes that can be accessed like regular files. FIFOs exist until they are deleted (for example with os.unlink()). Generally, FIFOs are used as rendezvous between “client” and “server” type processes: the server opens the FIFO for reading, and the client opens it for writing. Note that mkfifo() doesn’t open the FIFO — it just creates the rendezvous point.
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. Availability: Unix, Windows.
It is also possible to create temporary directories; see the tempfile module’s tempfile.mkdtemp() function.
Recursive directory creation function. Like mkdir(), but makes all intermediate-level directories needed to contain the leaf directory. Throws an error exception if the leaf directory already exists or cannot be created. 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 os.pardir.
This function handles UNC paths correctly.
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. Availability: Unix.
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.
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.
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.
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.
Perform a stat() system call on the given path. 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):
>>> import os
>>> statinfo = os.stat('somefile.txt')
>>> statinfo
(33188, 422511, 769, 1, 1032, 100, 926, 1105022698,1105022732, 1105022732)
>>> statinfo.st_size
926
>>>
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.
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.)
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.
Availability: Unix, Windows.
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.
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.
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))
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.
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.
Exit to the system with status n, without calling cleanup handlers, flushing stdio buffers, etc. Availability: Unix, Windows.
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.
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.
Send signal sig to the process pid. Constants for the specific signals available on the host platform are defined in the signal module. Availability: Unix.
Send the signal sig to the process group pgid. Availability: Unix.
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.
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.
Execute the command (a string) in a subshell. This is implemented by calling the Standard C function system(), and has the same limitations. Changes to sys.stdin, etc. are not reflected in the environment of the executed command.
On Unix, the return value is the exit status of the process encoded in the format specified for wait(). Note that POSIX does not specify the meaning of the return value of the C system() function, so the return value of the Python function is system-dependent.
On Windows, the return value is that returned by the system shell after running command, given by the Windows environment variable COMSPEC: on command.com systems (Windows 95, 98 and ME) this is always 0; on cmd.exe systems (Windows NT, 2000 and XP) this is the exit status of the command run; on systems using a non-native shell, consult your shell documentation.
Availability: Unix, Windows.
The subprocess module provides more powerful facilities for spawning new processes and retrieving their results; using that module is preferable to using this function. Use the subprocess module. Check especially the Replacing Older Functions with the subprocess Module section.
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.
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.
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. Availability: Unix.
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.
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.
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.