subprocess.Popen() stdout and stderr handling

Question

How can I handle a process stderr?

proc = subprocess.Popen('ll'.split(), stdout=subprocess.PIPE)

for i in proc.stdout:
   print(i)

Right now I am streaming the output but I am not sure how to properly deal with a potential error that might occur.

I wanted to use out, err = proc.communicate() but my out could be a very very very long string

Answer 1

If you know what error messages to expect, then one answer is to pass subprocess.STDOUT to the stderr argument of Popen , so that your stderr messages are in the stdout stream:

proc = subprocess.Popen('ll'.split(), stdout=subprocess.PIPE, stderr=subprocess.STDOUT)

for i in proc.stdout:
   print(i)
   # check for error message strings and do something with them

Or if you don't care about the stdout messages then just iterate over stderr instead:

dnull = open(os.devnull, 'w')
proc = subprocess.Popen('ll'.split(), stdout=dnull, stderr=subprocess.PIPE)

for i in proc.stderr:
   print(i)
   # check for error message strings and do something with them