How to mock stdout runtime attribute of subprocess.Popen call in Python 3 ?

PopenMock.py

#!/usr/bin/env python3


import os
import tempfile
import unittest
from subprocess import PIPE, Popen, check_output
from unittest.mock import patch


class YourClass(object):
    """
    This class contains a method in which we might want to mock the `Popen`
      call and not the `check_output` one.
    Nevertheless, the pipe uses a `.stdout` runtime attribute expecting a
      file-like object, so we have to go trough a manual workaround to mock it.
    Of course, you may want to do the same with `.stderr`, and it's possible ;)
    """
    def __init__(self):
        self.value = check_output(
            ['grep', '-E', 'regular|mocked'],
            stdin=Popen(['echo', 'STDOUT (regular)'],
                        stdout=PIPE).stdout  # The problem is here.
        ).decode().rstrip()


class PopenMock(unittest.TestCase):
    def setUp(self):
        # This temporary file will act as a standard stream pipe for `Popen`
        self.stdout_mock = tempfile.NamedTemporaryFile(delete=False)

    def tearDown(self):
        # At the end of the test, we'll close and remove the created file
        self.stdout_mock.close()
        os.remove(self.stdout_mock.name)

    @patch(
        '__main__.Popen'  # Please, take care of "patching" to the right place
    )
    def test(self, popen_mock):
        # We store some data into the fake pipe here
        self.stdout_mock.write(b'STDOUT (mocked)')

        # We have to rewind to the beginning of the file for the next reading
        self.stdout_mock.seek(0)

        # The fake standard stream attribute is set here
        popen_mock.return_value.stdout = self.stdout_mock

        # Run your test(s) !
        self.assertEqual(YourClass().value, 'STDOUT (mocked)')


if __name__ == '__main__':
    unittest.main()

In Python 2.7, its throwing the following error.

        process = Popen(stdout=PIPE, *popenargs, **kwargs)
>       output, unused_err = process.communicate()
E       ValueError: need more than 0 values to unpack

/usr/lib/python2.7/subprocess.py:567: ValueError

Could you give an example for 2.7?

Hey @sandeepkota,

As crazy as it may sound, GitHub has still not implemented a propoer Gists notifications engine, so your comment got lost... Sorry.

I've just tried to run this piece of code against Python2.7(.10), and with the following minor patch applied, it directly worked for me :

---  PopenMock_3.py	Sun Feb 17 10:39:14 2019
+++  PopenMock_2.py	Sun Feb 17 10:39:56 2019
@@ -1,11 +1,11 @@
-#!/usr/bin/env python3
+#!/usr/bin/env python2
 
 
 import os
 import tempfile
 import unittest
 from subprocess import PIPE, Popen, check_output
-from unittest.mock import patch
+from mock import patch
 
 
 class YourClass(object):

Have you fixed this issue since ?
Do you want me to look at your problem precisely ?

With your snippet, it looks like it was related to the passed arguments of Popen here.

Bye 👋