The os module
This module provides a unified interface to a number of operating system functions.
Working with files
The built-in open function lets you create, open, and modify files. This module adds those extra functions you need to rename and remove files:
# File: os-example-3.py import os import string def replace(file, search_for, replace_with): # replace strings in a text file back = os.path.splitext(file) + ".bak" temp = os.path.splitext(file) + ".tmp" try: # remove old temp file, if any os.remove(temp) except os.error: pass fi = open(file) fo = open(temp, "w") for s in fi.readlines(): fo.write(string.replace(s, search_for, replace_with)) fi.close() fo.close() try: # remove old backup file, if any os.remove(back) except os.error: pass # rename original to backup... os.rename(file, back) # ...and temporary to original os.rename(temp, file) # # try it out! file = "samples/sample.txt" replace(file, "hello", "tjena") replace(file, "tjena", "hello")
Working with directories
The os module also contains a number of functions that work on entire directories.
The listdir function returns a list of all filenames in a given directory. The current and parent directory markers used on Unix and Windows (. and ..) are not included in this list.
# File: os-example-5.py import os for file in os.listdir("samples"): print file
sample.au sample.jpg sample.wav ...
The getcwd and chdir functions are used to get and set the current directory:
# File: os-example-4.py import os # where are we? cwd = os.getcwd() print "1", cwd # go down os.chdir("samples") print "2", os.getcwd() # go back up os.chdir(os.pardir) print "3", os.getcwd()
1 /ematter/librarybook 2 /ematter/librarybook/samples 3 /ematter/librarybook
The makedirs and removedirs functions are used to create and remove directory hierarchies.
# File: os-example-6.py import os os.makedirs("test/multiple/levels") fp = open("test/multiple/levels/file", "w") fp.write("inspector praline") fp.close() # remove the file os.remove("test/multiple/levels/file") # and all empty directories above it os.removedirs("test/multiple/levels")
Note that removedirs removes all empty directories along the given path, starting with the last directory in the given path name. In contrast, the mkdir and rmdir functions can only handle a single directory level.
# File: os-example-7.py import os os.mkdir("test") os.rmdir("test") os.rmdir("samples") # this will fail
Traceback (innermost last): File "os-example-7", line 6, in ? OSError: [Errno 41] Directory not empty: 'samples'
To remove non-empty directories, you can use the rmtree function in the shutil module.
Working with file attributes
The stat function fetches information about an existing file. It returns a 9-tuple which contains the size, inode change timestamp, modification timestamp, and access privileges.
# File: os-example-1.py import os import time file = "samples/sample.jpg" def dump(st): mode, ino, dev, nlink, uid, gid, size, atime, mtime, ctime = st print "- size:", size, "bytes" print "- owner:", uid, gid print "- created:", time.ctime(ctime) print "- last accessed:", time.ctime(atime) print "- last modified:", time.ctime(mtime) print "- mode:", oct(mode) print "- inode/dev:", ino, dev # # get stats for a filename st = os.stat(file) print "stat", file dump(st) print # # get stats for an open file fp = open(file) st = os.fstat(fp.fileno()) print "fstat", file dump(st)
stat samples/sample.jpg - size: 4762 bytes - owner: 0 0 - created: Tue Sep 07 22:45:58 1999 - last accessed: Sun Sep 19 00:00:00 1999 - last modified: Sun May 19 01:42:16 1996 - mode: 0100666 - inode/dev: 0 2 fstat samples/sample.jpg - size: 4762 bytes - owner: 0 0 - created: Tue Sep 07 22:45:58 1999 - last accessed: Sun Sep 19 00:00:00 1999 - last modified: Sun May 19 01:42:16 1996 - mode: 0100666 - inode/dev: 0 0
Some fields don’t make sense on non-Unix platforms; for example, the (inode, dev) tuple provides a unique identity for each file on Unix, but can contain arbitrary data on other platforms.
The stat module contains a number of useful constants and helper functions for dealing with the members of the stat tuple. Some of these are shown in the examples below.
You can modify the mode and time fields using the chmod and utime functions:
# File: os-example-2.py import os import stat, time infile = "samples/sample.jpg" outfile = "out.jpg" # copy contents fi = open(infile, "rb") fo = open(outfile, "wb") while 1: s = fi.read(10000) if not s: break fo.write(s) fi.close() fo.close() # copy mode and timestamp st = os.stat(infile) os.chmod(outfile, stat.S_IMODE(st[stat.ST_MODE])) os.utime(outfile, (st[stat.ST_ATIME], st[stat.ST_MTIME])) print "original", "=>" print "mode", oct(stat.S_IMODE(st[stat.ST_MODE])) print "atime", time.ctime(st[stat.ST_ATIME]) print "mtime", time.ctime(st[stat.ST_MTIME]) print "copy", "=>" st = os.stat(outfile) print "mode", oct(stat.S_IMODE(st[stat.ST_MODE])) print "atime", time.ctime(st[stat.ST_ATIME]) print "mtime", time.ctime(st[stat.ST_MTIME])
original => mode 0666 atime Thu Oct 14 15:15:50 1999 mtime Mon Nov 13 15:42:36 1995 copy => mode 0666 atime Thu Oct 14 15:15:50 1999 mtime Mon Nov 13 15:42:36 1995
Working with processes
The system function runs a new command under the current process, and waits for it to finish.
# File: os-example-8.py import os if os.name == "nt": command = "dir" else: command = "ls -l" os.system(command)
-rwxrw-r-- 1 effbot effbot 76 Oct 9 14:17 README -rwxrw-r-- 1 effbot effbot 1727 Oct 7 19:00 SimpleAsyncHTTP.py -rwxrw-r-- 1 effbot effbot 314 Oct 7 20:29 aifc-example-1.py -rwxrw-r-- 1 effbot effbot 259 Oct 7 20:38 anydbm-example-1.py ...
The command is run via the operating system’s standard shell, and returns the shell’s exit status. Under Windows 95/98, the shell is usually command.com whose exit status is always 0.
Warning: Since os.system passes the command on to the shell as is, it can be dangerous to use if you don’t check the arguments carefully (consider running os.system(“viewer %s” % file) with the file variable set to “sample.jpg; rm -rf $HOME”). When unsure, it’s usually better to use exec or spawn instead (see below).
The exec function starts a new process, replacing the current one (“go to process”, in other words). In the following example, note that the “goodbye” message is never printed:
# File: os-exec-example-1.py import os import sys program = "python" arguments = ["hello.py"] print os.execvp(program, (program,) + tuple(arguments)) print "goodbye"
hello again, and welcome to the show
Python provides a whole bunch of exec functions, with slightly varying behavior. The above example uses execvp, which searches for the program along the standard path, passes the contents of the second argument tuple as individual arguments to that program, and runs it with the current set of environment variables. See the Python Library Reference for more information on the other seven ways to call this function.
Under Unix, you can call other programs from the current one by combining exec with two other functions, fork and wait. The former makes a copy of the current process, the latter waits for a child process to finish.
# File: os-exec-example-2.py import os import sys def run(program, *args): pid = os.fork() if not pid: os.execvp(program, (program,) + args) return os.wait() run("python", "hello.py") print "goodbye"
hello again, and welcome to the show goodbye
The fork returns zero in the new process (the return from fork is the first thing that happens in that process!), and a non-zero process identifier in the original process. Or in other words, “not pid” is true only if we’re in the new process.
fork and wait are not available on Windows, but you can use the spawn function instead. Unfortunately, there’s no standard version of spawn that searches for an executable along the path, so you have to do that yourself:
# File: os-spawn-example-1.py import os import string def run(program, *args): # find executable for path in string.split(os.environ["PATH"], os.pathsep): file = os.path.join(path, program) + ".exe" try: return os.spawnv(os.P_WAIT, file, (file,) + args) except os.error: pass raise os.error, "cannot find executable" run("python", "hello.py") print "goodbye"
hello again, and welcome to the show goodbye
You can also use spawn to run other programs in the background. The following example adds an optional mode argument to the run function; when set to os.P_NOWAIT, the script doesn’t wait for the other program to finish.
The default flag value os.P_WAIT tells spawn to wait until the new process is finished. Other flags include os.P_OVERLAY which makes spawn behave like exec, and os.P_DETACH which runs the new process in the background, detached from both console and keyboard.
# File: os-spawn-example-2.py import os import string def run(program, *args, **kw): # find executable mode = kw.get("mode", os.P_WAIT) for path in string.split(os.environ["PATH"], os.pathsep): file = os.path.join(path, program) + ".exe" try: return os.spawnv(mode, file, (file,) + args) except os.error: pass raise os.error, "cannot find executable" run("python", "hello.py", mode=os.P_NOWAIT) print "goodbye"
goodbye hello again, and welcome to the show
The following example provides a spawn method that works on either platform:
# File: os-spawn-example-3.py import os import string if os.name in ("nt", "dos"): exefile = ".exe" else: exefile = "" def spawn(program, *args): try: # check if the os module provides a shortcut return os.spawnvp(program, (program,) + args) except AttributeError: pass try: spawnv = os.spawnv except AttributeError: # assume it's unix pid = os.fork() if not pid: os.execvp(program, (program,) + args) return os.wait() else: # got spawnv but no spawnp: go look for an executable for path in string.split(os.environ["PATH"], os.pathsep): file = os.path.join(path, program) + exefile try: return spawnv(os.P_WAIT, file, (file,) + args) except os.error: pass raise IOError, "cannot find executable" # # try it out! spawn("python", "hello.py") print "goodbye"
hello again, and welcome to the show goodbye
The above example first attempts to call a function named spawnvp. If that doesn’t exist (it doesn’t, in 2.0 and earlier), the function looks for a function named spawnv and searches the path all by itself. As a last resort, it falls back on exec and fork.
Working with daemon processes
On Unix, fork can also be used to turn the current process into a background process (a “daemon”). Basically, all you need to do is to fork off a copy of the current process, and terminate the original process:
# File: os-example-14.py import os import time pid = os.fork() if pid: os._exit(0) # kill original print "daemon started" time.sleep(10) print "daemon terminated"
However, it takes a bit more work to create a real daemon. First, call setpgrp to make the new process a “process group leader”. Otherwise, signals sent to a (by that time) unrelated process group might cause problems in your daemon:
It’s also a good idea to remove the user mode mask, to make sure files created by the daemon actually gets the mode flags specified by the program:
Then, you should redirect the stdout/stderr files, instead of just closing them. If you don’t do this, you may get unexpected exceptions the day some of your code tries to write something to the console via stdout or stderr.
class NullDevice: def write(self, s): pass sys.stdin.close() sys.stdout = NullDevice() sys.stderr = NullDevice()
In other words, while Python’s print and C’s printf/fprintf won’t crash your program if the devices have been disconnected, sys.stdout.write() happily throws an IOError exception when the application runs as a daemon. But your program works just fine when running in the foreground…
By the way, the _exit function used in the examples above terminates the current process. In contrast to sys.exit, this works also if the caller happens to catch the SystemExit exception:
# File: os-example-9.py import os import sys try: sys.exit(1) except SystemExit, value: print "caught exit(%s)" % value try: os._exit(2) except SystemExit, value: print "caught exit(%s)" % value print "bye!"