How to use the subprocess Popen.communicate() method?

Question

I'm trying to get the standard output of a bash command as a string in Python. Following Popen documentation, I've tried:

import subprocess

p = subprocess.Popen(["echo", "hello"])
stdoutdata, stderrdata = p.communicate()

print stdoutdata

Running this script yields the following output:

hello
None
[Finished in 0.0s]

So although the output is getting printed by Python, the stdoutdata variable is None, and not "hello" as I would like. How can I make it so?

Answer 1

You're not providing any stdout to the Popen constructor, the default functionality simply writes the output to parent's stdout handle. Hence you're seeing it being printed in your shell.

Quoting from Popen's docs:

With the default settings of None, no redirection will occur; the child’s file handles will be inherited from the parent.

---

To populate stdout in resulting tuple use subprocess.PIPE as stdout.

Quoting from docs:

To get anything other than None in the result tuple, you need to give stdout=PIPE and/or stderr=PIPE too.

>>> import subprocess
>>> p = subprocess.Popen(["echo", "hello"], stdout=subprocess.PIPE)
>>> p.communicate()
('hello\n', None)

Answer 2

You need to pass stdout, stderr flags to Popen constructor.
Per default they are set to None, resulting in Popen is not capturing them.

cmd = subprocess.Popen(["echo", "hello"], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
out, err = cmd.communicate()
# retCode = cmd.returncode
# retCode != 0, indicates an error occured in execution.
print (out)
>>> b'hello\n'

Answer 3

It seems like the subprocess.check_output method is what I need:

import subprocess
output = subprocess.check_output(["echo", "hello"])

The output is now 'hello\n' (including a newline character) as I would expect.