‘yes’ reporting error with subprocess communicate()

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The issue is that subprocess module before Python 3.2+ doesn’t restore SIGPIPE signal handler to default action. That is why you get EPIPE write error instead.

In Python 3.2+

>>> from subprocess import check_output
>>> check_output("yes | head -3", shell=True)
b'y\ny\ny\n'

yes is killed by SIGPIPE when head exits.

In Python 2:

>>> from subprocess import check_output
>>> check_output("yes | head -3", shell=True)
yes: standard output: Broken pipe
yes: write error
'y\ny\ny\n'

yes got EPIPE write error. It is safe to ignore the error. It communicates the same information as SIGPIPE.

To workaround the problem, you could emulate restore_signals in Python 2 using preexec_fn parameter :

>>> from subprocess import check_output
>>> import signal
>>> def restore_signals(): # from http://hg.python.org/cpython/rev/768722b2ae0a/
...     signals = ('SIGPIPE', 'SIGXFZ', 'SIGXFSZ')
...     for sig in signals:
...         if hasattr(signal, sig):
...            signal.signal(getattr(signal, sig), signal.SIG_DFL)
... 
>>> check_output("yes | head -3", shell=True, preexec_fn=restore_signals)
'y\ny\ny\n'

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