The os module

This module provides a unified interface to a number of operating system functions.

Most of the functions in this module are implemented by platform specific modules, such as posix and nt. The os module automatically loads the right implementation module when it is first imported.

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:

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.

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.

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()[0]

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:

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.

The following example provides a spawn method that works on either platform:

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:

os.setpgrp()

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:

os.umask(0)

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!"

caught exit(1)