Percentage of Memory Used with Sar and Python for Linux

I needed to get the Percent Memory Used for a Linux systems using sar.
Linux grabs all the memory even though it does not use it all so it is takes a bit more work to figure out how memory is being used.
The formula is:
((kbmemused - kbbuffers - kbcached) / (kbmemfree + kbmemused)) * 100

#!/usr/bin/env python2.7
import re
import subprocess

mcmd = "sar -r -f /xactly/apps/sar/sjcxtlyapp01s/sa20 |grep Average"

sep = re.compile('[\s]+')
sar = subprocess.Popen([mcmd],
    stdin=subprocess.PIPE,
    stdout=subprocess.PIPE,
    stderr=subprocess.PIPE,
    shell=True)
string =  sep.split(str(sar.stdout.readlines()))
memfree = "%0.2f" %  (float(string[1]) / 1024)
memused = "%0.2f" %  (float(string[2]) / 1024)
buffers = "%0.2f" %  (float(string[4]) / 1024)
cached =  "%0.2f" % (float(string[5]) / 1024)
percent_used = "%0.2f" %  (((float(memused) - float(buffers) - float(cached))  / (float(memfree) + float(memused))) * 100)
print percent_used