system calls to sh

[Kia A. Arab]

I can't seem to affect environment variables using os.system calls. I
don't know if this makes a difference, but the variables are set in c
shell. I try to unset them from a python script like so:

os.system('unset MY_VAR')
os.system('export MY_VAR')
os.system('echo $MY_VAR')	# not unset!