Real-time intercepting of stdout from another process in Python

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Looping over a file unavoidably buffers things in pretty large chunks — a known issue with all Python 2.* implementations. It works as you intend in Python 3.1, with the final loop being slightly different:

for line in proc.stdout:
    print(">>> " + str(line.rstrip()))

If upgrading to Python 3.1 is impractical (and I know it will often be!), go the other way and write the loop in an old-fashioned manner — the following version of the loop does work as you intend in Python 2.*:

while True:
    line = proc.stdout.readline()
    if not line:
        break
    print ">>> " + line.rstrip()

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