I want to capture the stdout stream of a shell command in a python (3) script, and being able, at the same time, to check the return code of the shell command if it returns an error (that is, if its return code is not 0).

subprocess.check_output seems to be the appropriate method to do this. From subprocess‘s man page:

check_output(*popenargs, **kwargs)
    Run command with arguments and return its output as a byte string.

    If the exit code was non-zero it raises a CalledProcessError.  The
    CalledProcessError object will have the return code in the returncode
    attribute and output in the output attribute.

Still, I don’t succeed to obtain the return code from the shell command when it fails. My code looks like this:

import subprocess
failing_command=['ls', 'non_existent_dir']

try:
    subprocess.check_output(failing_command)
except:
    ret = subprocess.CalledProcessError.returncode  # <- this seems to be wrong
    if ret in (1, 2):
        print("the command failed")
    elif ret in (3, 4, 5):
        print("the command failed very much")

This code raises an exception in the handling of the exception itself:

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
AttributeError: type object 'CalledProcessError' has no attribute 'returncode'

I admit I don’t know where I am wrong.

To get both the process output and the returned code:

from subprocess import Popen, PIPE

p = Popen(["ls", "non existent"], stdout=PIPE)
output = p.communicate()[0]
print(p.returncode)

subprocess.CalledProcessError is a class. To access returncode use the exception instance:

from subprocess import CalledProcessError, check_output

try:
    output = check_output(["ls", "non existent"])
    returncode = 0
except CalledProcessError as e:
    output = e.output
    returncode = e.returncode

print(returncode)

Most likely my answer is no longer relevant, but I think it may be solved with this code:

import subprocess
failing_command='ls non_existent_dir'

try:
    subprocess.check_output(failing_command, shell=True, stderr=subprocess.STDOUT)
except subprocess.CalledProcessError as e:
    ret =   e.returncode 
    if ret in (1, 2):
        print("the command failed")
    elif ret in (3, 4, 5):
        print("the command failed very much")