Mocking a subprocess call in Python

Question

I have a method - run_script() - I would like to test. Specifically, I want to test that a call to subprocess.Popen occurs. It would be even better to test that subprocess.Popen is called with certain parameters. When I run the test however I get TypeError: 'tuple' object is not callable.

How can I test my method to ensure that subprocess is actually being called using mocks?

@mock.patch("subprocess.Popen")
def run_script(file_path):
    process = subprocess.Popen(["myscript", -M, file_path], stdout=subprocess.PIPE)
    output, err = process.communicate()
    return process.returncode


def test_run_script(self, mock_subproc_popen):
    mock_subproc_popen.return_value = mock.Mock(
        communicate=("ouput", "error"), returncode=0
    )
    am.account_manager("path")
    self.assertTrue(mock_subproc_popen.called)

Answer 1

It seems unusual to me that you use the patch decorator over the run_script function, since you don't pass a mock argument there.

How about this:

from unittest import mock
import subprocess

def run_script(file_path):
    process = subprocess.Popen(["myscript", -M, file_path], stdout=subprocess.PIPE)
    output, err = process.communicate()
    return process.returncode


@mock.patch("subprocess.Popen")
def test_run_script(self, mock_subproc_popen):
    process_mock = mock.Mock()
    attrs = {"communicate.return_value": ("output", "error")}
    process_mock.configure_mock(**attrs)
    mock_subproc_popen.return_value = process_mock
    am.account_manager("path")  # this calls run_script somewhere, is that right?
    self.assertTrue(mock_subproc_popen.called)

Right now, your mocked subprocess.Popen seems to return a tuple, causeing process.communicate() to raise TypeError: 'tuple' object is not callable.. Therefore it's most important to get the return_value on mock_subproc_popen just right.