check output python

1import subprocess
2py2output = subprocess.check_output(['python', 'py2.py', '-i', 'test.txt'])
3print('py2 said:', py2output)
4

1The right answer (using Python 2.7 and later, since check_output() was introduced then) is:
2
3py2output = subprocess.check_output(['python','py2.py','-i', 'test.txt'])
4To demonstrate, here are my two programs:
5
6py2.py:
7
8import sys
9print sys.argv
10py3.py:
11
12import subprocess
13py2output = subprocess.check_output(['python', 'py2.py', '-i', 'test.txt'])
14print('py2 said:', py2output)
15Running it:
16
17$ python3 py3.py
18py2 said: b"['py2.py', '-i', 'test.txt']\n"
19Here's what's wrong with each of your versions:
20
21py2output = subprocess.check_output([str('python py2.py '),'-i', 'test.txt'])
22First, str('python py2.py') is exactly the same thing as 'python py2.py'—you're taking a str, and calling str to convert it to an str. This makes the code harder to read, longer, and even slower, without adding any benefit.
23
24More seriously, python py2.py can't be a single argument, unless you're actually trying to run a program named, say, /usr/bin/python\ py2.py. Which you're not; you're trying to run, say, /usr/bin/python with first argument py2.py. So, you need to make them separate elements in the list.
25
26Your second version fixes that, but you're missing the ' before test.txt'. This should give you a SyntaxError, probably saying EOL while scanning string literal.
27
28Meanwhile, I'm not sure how you found documentation but couldn't find any examples with arguments. The very first example is:
29
30>>> subprocess.check_output(["echo", "Hello World!"])
31b'Hello World!\n'
32That calls the "echo" command with an additional argument, "Hello World!".
33
34Also:
35
36-i is a positional argument for argparse, test.txt is what the -i is
37
38I'm pretty sure -i is not a positional argument, but an optional argument. Otherwise, the second half of the sentence makes no sense.