How to get notified on file conversion in command line

Per Python documentation, subprocess.call should be blocking and wait for the subprocess to complete. In this code I am trying to convert few xls files to a new format by calling Libreoffice on command line. I assumed that the call to subprocess call is blocking but seems like I need to add an artificial delay after each call otherwise I miss few files in the out directory.

what am I doing wrong? and How to get notified when the file conversion is complete?

from subprocess import call

for i in range(0,len(sorted_files)):
            args = ['libreoffice', '-headless', '-convert-to',
                    'xls', "%s/%s.xls" %(sorted_files[i]['filename'],sorted_files[i]['filename']), '-outdir', 'out']
            call(args)
            var = raw_input("Enter something: ") # if comment this line I dont get all the files in out directory

This question has been answered over at Stack Overflow. The essence is that the libreoffice command is a controller for the real program soffice.bin, thus the call command quits once libreoffice quits, rather than waiting for the LO process to finish. Workaround is to make use of the unoconv tool which already offers python UNO API access.