Here is a short recipe to get sys­tem uptime in Python. Tested so far on Ubuntu linux and Solaris. Obvi­ously won't work on Windows.

gist on Github for sys_uptime.py

import subprocess

def uptime():
    raw = subprocess.check_output('uptime').replace(',','')
    days = int(raw.split()[2])
    if 'min' in raw:
    	hours = 0
    	minutes = int(raw[4])
    else:
    	hours, minutes = map(int,raw.split()[4].split(':'))
    totalsecs = days*24*60*60 + hours*60*60 + minutes*60
    return totalsecs

print 'System uptime of %d seconds' % (uptime())