Mocking a subprocess call in Python

Question

I have a method (run_script) would like to test. Specifically I want to test that a call to subprocess.Popenoccurs. 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:

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.

Answer 2

There exists the testfixtures package that you can make use of.

Here's an example on using the mock Popen:

from unittest import TestCase  from testfixtures.mock import call from testfixtures import Replacer, ShouldRaise, compare from testfixtures.popen import MockPopen, PopenBehaviour   class TestMyFunc(TestCase):      def setUp(self):         self.Popen = MockPopen()         self.r = Replacer()         self.r.replace(dotted_path, self.Popen)         self.addCleanup(self.r.restore)      def test_example(self):         # set up         self.Popen.set_command('svn ls -R foo', stdout=b'o', stderr=b'e')          # testing of results         compare(my_func(), b'o')          # testing calls were in the right order and with the correct parameters:         process = call.Popen(['svn', 'ls', '-R', 'foo'], stderr=PIPE, stdout=PIPE)         compare(Popen.all_calls, expected=[             process,             process.communicate()         ])      def test_example_bad_returncode(self):         # set up         Popen.set_command('svn ls -R foo', stdout=b'o', stderr=b'e',                           returncode=1)          # testing of error         with ShouldRaise(RuntimeError('something bad happened')):             my_func()