python - Get exit code and stderr from subprocess call

Question

I read up on the functions provided by subprocess - call, check_call, check_output, and understand how each works and differs in functionality from one another. I am currently using check_output, so I can have access to the stdout, and used "try block" to catch the exception, as follows:

# "cmnd" is a string that contains the command along with it's arguments. 
try:
    cmnd_output = check_output(cmnd, stderr=STDOUT, shell=True, timeout=3, universal_newlines=True);                         
except CalledProcessError:                                                                                                   
    print("Status : FAIL")                                                                                                   
print("Output: 
{}
".format(cmnd_output))                                                                                  

The issue I am running into is when an exception is thrown, "cmnd_output" is not initialized and don't have access to stderr, and I get the following error message:

print("Output: 
{}
".format(cmnd_output))
UnboundLocalError: local variable 'cmnd_output' referenced before assignment

I think thats because the exception causes the "check_output" to bail immediately without any further processing, aka assignment to "cmnd_output", in the try block. Please correct me if I am wrong.

Is there any way I can get access to stderr (it's ok if it's sent to stout) and have access to the exit code. I can manually check for pass/fail based on exit code with out the exception being throuwn.

Thank you, Ahmed.

Answer 1

Try this version:

import subprocess
try:
    output = subprocess.check_output(
        cmnd, stderr=subprocess.STDOUT, shell=True, timeout=3,
        universal_newlines=True)
except subprocess.CalledProcessError as exc:
    print("Status : FAIL", exc.returncode, exc.output)
else:
    print("Output: 
{}
".format(output))

This way you will print the output only if the call was successful. In case of a CalledProcessError you print the return code and the output.