[python] How do you use subprocess.check_output() in Python?

I have found documentation about subprocess.check_output() but I cannot find one with arguments and the documentation is not very in depth. I am using Python 3 (but am trying to run a Python 2 file through Python 3)

I am trying to run this command: python py2.py -i test.txt

-i is a positional argument for argparse, test.txt is what the -i is, py2.py is the file to run

I have tried a lot of (non working) variations including: py2output = subprocess.check_output([str('python py2.py '),'-i', 'test.txt'])

py2output = subprocess.check_output([str('python'),'py2.py','-i', test.txt'])

Since Python 3.5, subprocess.run() is recommended over subprocess.check_output():

>>> subprocess.run(['cat','/tmp/text.txt'], stdout=subprocess.PIPE).stdout
b'First line\nSecond line\n'

Since Python 3.7, instead of the above, you can use capture_output=true parameter to capture stdout and stderr:

>>> subprocess.run(['cat','/tmp/text.txt'], capture_output=True).stdout
b'First line\nSecond line\n'

Also, you may want to use universal_newlines=True or its equivalent since Python 3.7 text=True to work with text instead of binary:

>>> stdout = subprocess.run(['cat', '/tmp/text.txt'], capture_output=True, text=True).stdout
>>> print(stdout)
First line
Second line

See subprocess.run() documentation for more information.

Adding on to the one mentioned by @abarnert

a better one is to catch the exception

import subprocess
try:
    py2output = subprocess.check_output(['python', 'py2.py', '-i', 'test.txt'],stderr= subprocess.STDOUT)  
    #print('py2 said:', py2output)
    print "here"
except subprocess.CalledProcessError as e:
    print "Calledprocerr"

this stderr= subprocess.STDOUT is for making sure you dont get the filenotfound error in stderr- which cant be usually caught in filenotfoundexception, else you would end up getting

python: can't open file 'py2.py': [Errno 2] No such file or directory

Infact a better solution to this might be to check, whether the file/scripts exist and then to run the file/script