How to catch a line with Popen

[TheSaint]

Hello.
I'm looking into subprocess.Popen docs.
I've launch the program with its arguments and that's smooth. I'm expecting 
to read the output by *comunicate()* at every line that prgram may blow 
during the process, but the output is given only when the child process is 
ended.
I'd like to process the lines to display an information in percentage during 
the running time of the child. Sorry but I'm poor of know-how, so I'm stuck 
over to find a clue.

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goto /dev/null