running pg_dump from python

I ahve the following python file that I am running as an external method
in Zope.

def backup():
import os
os.popen("c:/scholarpack/postgres/bin/pg_dump scholarpack >
c:/scholarpack/ancillary/scholarpack.sql")
data=open('c:/scholarpack/ancillary/scholarpack.sql','r')
r=data.read()
data.close
return r

However, when I run this script it creates the file scholarpack.sql but
the file is empty.
Any help much appreciated.
Regards
Garry