>>> import subprocess as subp
>>> 
>>> p = subp.Popen('echo x | cat -n', shell=True, stdout=subp.PIPE)
>>> out, err = p.communicate()
>>> out, err, p.returncode
('     1\tx\n', None, 0)
>>>

import subprocess
infoHDD []
proc = subprocess.Popen('sdparm --quiet --page=sn /dev/sda', shell=True, stdout=subprocess.PIPE)
while True:
    line = proc.stdout.readline()
    if not line:
        break
    infoHDD.append(line)
print (infoHDD[1])

import subprocess
proc = subprocess.Popen('sdparm --quiet --page=sn /dev/sda', shell=True, stdout=subprocess.PIPE)
proc.wait() # ждем завершения работы дочерней программы
infoHDD=proc.stdout.readlines()[1]
print (infoHDD)