Why does my Python3 script balk at piping its output to head or tail (sys module)?

Question

I have a Python3 script that writes its output to stdout, but it complains when I pipe that output into head or tail. Note in the sample output below that it sort of works,

Answer 1

./script.py '../Testdata/*indels.ss' -m 5 | awk 'NR >= 3 {exit} 1'

would show the same behavior as head -2.

You can turn set the SIGPIPE handler to one which quietly kills your program instead:

import signal
signal.signal(signal.SIGPIPE, signal.SIG_DFL)

Answer 2

I'll cite from here:

If a sequence of commands appears in a pipeline, and one of the

reading commands finishes before the writer has finished, the

writer receives a SIGPIPE signal.

That's what head does. Your script hasn't finished writing, but head is already done, so stdout is closed, hence the exception.