python mock side-effect return value

Question

I am trying to test some code that makes an external call. I want to mock that call out. The call takes keyword args, so I wrote this little helper function in my test:

def mock_function(*args, **kwargs)
    io_obj = StringIO()
    for k,v in kwargs.iteritems():
        io_obj.write("{}: {}\n".format(k, v)
    print "\n{}".format(io_obj.getvalue()) # for testing purposes
    return io_obj

in my setUp function for the test class, I have this:

@patch('function_to_test')
def setUp(self, mock_dude):
    self.mock_client = mock_dude.return_value
    self.mock_client.function_to_test.side_effect = mock_function
    self.client = ClientClass()

in my test function, I am calling the function that calls the external function. I get the printout from mock_function, so I know that I am mocking the function correctly. My question is this:

How can I get at the io_obj that is created in mock_function? My external function doesn't return anything.

Answer 1

The Mock object actually captures the arguments it's called with, so you don't need to write your own function to do that. You can access the arguments directly using Mock.call_args , or assert that the mock was called with certain arguments using assert_called_with .

Example:

>>> m = mock.Mock()
>>> m(1,2,3)
<Mock name='mock()' id='139905514719504'>
>>> m.call_args
call(1, 2, 3)
>>> m.assert_called_with(1,2,4)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib64/python2.6/site-packages/mock.py", line 835, in assert_called_with
    raise AssertionError(msg)
AssertionError: Expected call: mock(1, 2, 4)
Actual call: mock(1, 2, 3)