steve steve - 3 months ago 8
Python Question

Mocking a subprocess call in Python

I have a method (

run_script
) 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

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.