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When not using a timeout, communicate will raise a BrokenPipeError if the command returns an error code. For example:
sp = subprocess.Popen('cat --not-an-option', shell=True, stdin=subprocess.PIPE)
time.sleep(.2)
sp.communicate(b'hi\n')
This isn't consistent with the behavior of communicate with a timeout, which doesn't raise the exception. Moreover, there is even a comment near the point of the exception stating that communicate must ignore BrokenPipeError:
 def _stdin_write(self, input):
 if input:
 try:
 self.stdin.write(input)
 except BrokenPipeError:
 # communicate() must ignore broken pipe error
 pass
 ....
 self.stdin.close()
Obviously, the problem is that self.stdin.close() is outside the except clause.