shutting down computer (linux) using python

The best way to shutdown a system is to use the following codes

import os
os.system('systemctl poweroff') 

Many of the linux distributions out there require super user privileges to execute shutdown or halt, but then, how come that if you're sitting on your computer you can power it off without being root? You open a menu, hit Shutdown and it shutdowns without you becoming root, right?

Well... the rationale behind this is that if you have physical access to the computer, you could pretty much pull the power cord and power it off anyways, so nowadays, many distributions allow power-off though access to the local System Bus accessible through dbus. Problem with dbus (or the services exposed through it, rather)? It's constantly changing. I'd recommend installing a dbus viewer tool such as D-feet (be advised: it's still pretty hard to visualize, but it may help)

Take a look to these Dbus shutdown scripts.

If you still have HAL in your distrubution (is on the way to being deprecated) try this:

import dbus
sys_bus = dbus.SystemBus()
hal_srvc = sys_bus.get_object('org.freedesktop.Hal',
                              '/org/freedesktop/Hal/devices/computer')
pwr_mgmt =  dbus.Interface(hal_srvc,
                'org.freedesktop.Hal.Device.SystemPowerManagement')
shutdown_method = pwr_mgmt.get_dbus_method("Shutdown")
shutdown_method()

This works on a Ubuntu 12.04 (I just powered off my computer to make sure it worked). If you have something newer... well, it may not work. It's the downside of this method: it is very distribution specific.

You might have to install the dbus-python package for this to work (http://dbus.freedesktop.org/doc/dbus-python/doc/tutorial.html)

UPDATE 1:

I've been doing a little bit of research and it looks like this is done in newer Ubuntu versions through ConsoleKit. I've tested the code below in my Ubuntu 12.04 (which has the deprecated HAL and the newer ConsoleKit) and it did shut my computer off:

>>> import dbus
>>> sys_bus = dbus.SystemBus()
>>> ck_srv = sys_bus.get_object('org.freedesktop.ConsoleKit',
                                '/org/freedesktop/ConsoleKit/Manager')
>>> ck_iface = dbus.Interface(ck_srv, 'org.freedesktop.ConsoleKit.Manager')
>>> stop_method = ck_iface.get_dbus_method("Stop")
>>> stop_method()

UPDATE 2:

Probably why can you do this without being root deserves a bit of a wider explanation. Let's focus on the newer ConsoleKit (HAL is way more complicated and messy, IMHO).

The ConsoleKit is a service running as root in your system:

borrajax@borrajax:/tmp$ ps aux|grep console-kit
root 1590  0.0  0.0 1043056 3876 ? Sl   Dec05   0:00 /usr/sbin/console-kit-daemon --no-daemon

Now, d-bus is just a message passing system. You have a service, such as ConsoleKit that exposes an interface to d-bus. One of the methods exposed is the Stop (shown above). ConsoleKit's permissions are controlled with PolKit, which (despite on being based on regular Linux permissions) offers a bit of a finer grain of control for "who can do what". For instance, PolKit can say things like "If the user is logged into the computer, then allow him to do something. If it's remotely connected, then don't.". If PolKit determines that your user is allowed to call ConsoleKit's Stop method, that request will be passed by (or through) d-bus to ConsoleKit (which will subsequently shutdown your computer because it can... because it worth's it... because it's root)

Further reading:

• What are ConsoleKit and PolicyKit? How do they work?
• ArchWiki PolKit

To summarize: You can't switch a computer off without being root. But you can tell a service that is running as root to shutdown the system for you.

UPDATE 3:

On December 2021, seven years after the original answer was written I had to do this again. This time, in a Ubuntu 18.04.

Unsurprisingly, things seem to have changed a bit:

• The PowerOff functionality seems to be handled via a new org.freedesktop.login1 service, which is part of the """new""" _{(cough! cough!)} SystemD machinery.
• The dbus Python package seems to have been deprecated and/or considered "legacy". There is, however, a new PyDbus library to be used instead.

So we can still power off machines with an unprivileged script:

#! /usr/bin/python3
from pydbus import SystemBus
bus = SystemBus()

proxy = bus.get('org.freedesktop.login1', '/org/freedesktop/login1')
if proxy.CanPowerOff() == 'yes':
    proxy.PowerOff(False)  # False for 'NOT interactive'

Update 3.1:

It looks like it's not as new as I thought X-D

There's already an answer by @Roeften in this very same thread.

BONUS:

I read in one of your comments that you wanna switch the computer off after a time consuming task to prevent it from overheating... Did you know that you can probably power it on at a given time using RTC? (See this and this) Pretty cool, uh? (I got so excited when I found out I could do this... ) :-D

import os
os.system("shutdown now -h")

execute your script with root privileges.

any way to shut down...without elevated privileges?

No, there isn't (fortunately!).

Keep in mind that you can use several system features to make privilege escalation for normal users easier:

• sudo
• setuid
• setcap