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