problem capturing output from dos program

[Guy Robinson]

Hello,

This works and correctly outputs the file out2.dwf

import os
test = os.popen("rewrite.exe test.dwf out2.dwf /a")

for L in test.readlines():
     print L

this prints the statistics from the translation.

The things is I want to capture the output file for further processing. 
I tried this:

infile = 'test.dwf'
outfile = 'out3.dwf'

os.spawnl(os.P_WAIT,'rewrite.exe',infile,outfile,'/a')

And it doesn't work, is not spawnl correct method. What am I doing 
wrong? The dos box disappears too quickly to see what the error is and 
ideally I don't want to see a dos box anyway.

Regards,

Guy