Python Unit Testing Mocking Popen and Popen.communicate

Here is the way to mock Popen and Popen.communicate in a Python unit test:

Python
import unittest
import mock

class TestPopen(unittest.TestCase):

    @mock.patch('subprocess.Popen')
    def test_communicate(self, mock_popen):
        # Create a mock object for the `Popen` object.
        mocked_popen = mock_popen.return_value

        # Set the `communicate` method on the mock object to return a
        # specific value.
        mocked_popen.communicate.return_value = ('stdout', 'stderr')

        # Call the `communicate` method on the mock object.
        output, error = mocked_popen.communicate()

        # Assert that the `communicate` method was called with the
        # expected arguments.
        mock_popen.communicate.assert_called_once_with()

        # Assert that the `communicate` method returned the expected
        # value.
        self.assertEqual(output, 'stdout')
        self.assertEqual(error, 'stderr')

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

The mock library can be used to create a mock object that simulates the behavior of the Popen object. The mock.patch decorator can be used to patch the Popen object in the test case, so that when the Popen object is called in the test case, it will actually call the mock object instead. This allows us to test our code without actually calling the Popen object, which can be helpful for isolating and debugging our code.

We then set the communication method on the mock object to return a specific value. This means that when the communication method is called on the mock object in the test case, it will return the value that we specified.

Finally, we call the communicate method on the mock object and assert that it was called with the expected arguments and that it returned the expected value.