*SVNLOOK_PATH=os.system('which svnlook')*
You've been told what's wrong with that. But instead of fixing how to invoke "which", use distutils.spawn.find_executable instead:
py> from distutils.spawn import find_executable py> find_executable('svnlook') 'c:\\apps\\svn\\bin\\svnlook.exe' -- Gabriel Genellina -- http://mail.python.org/mailman/listinfo/python-list