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I often run shell commands from python with Popen, but not often enough do I set up error handline for these subprocesses. It's not too hard, but it can be a bit awkward if you don't do it enough.
Using Popen
import subprocess from subprocess import Popen # this will run the shell command `cat me` and capture stdout and stderr proc = Popen(["cat", "me"], stdout=subprocess.PIPE, stderr=subprocess.PIPE) # this will wait for the process to finish. proc.wait()
reading from stderr
To get the stderr we must get it from the proc, read it, and decode the bystring. Note that we can only get the stderr object once, so if you want to do more than just read it you will need to store a copy of it.
proc.stderr.read().decode()
Better Exception
Now that we can read the stderr we can make better error tracking for the
user so they can see what to do to resolve the issue rather than blindly
failing.
err_message = proc.stderr.read().decode() if proc.returncode != 0: # the process was not successful if "No such file" in err_message: raise FileNotFoundError('No such file "me"')