Mocking the Presenter

The view should be so simple that it does not require testing. However, the presenter contains logic and should therefore be tested. When testing the presenter we do not want to create the actual GUI, instead we just want to ensure that the expected calls are made, this is done via mocking see unittest docs for a detailed discussion. Here we will have a brief discussion of the main points.

First are the import statements

import sys
from presenter import Presenter
from view import View

import unittest
from unittest import mock

The test class is then initialised:

class PresenterTest(unittest.TestCase):
    def setUp(self):
        self._view = mock.create_autospec(View)

        # mock view
        self._view._button_clicked = mock.Mock()
        self._view.get_value = mock.Mock(return_value=3.14)

        self._presenter = Presenter(self._view)

create_autospec mocks the class contained within the brackets. We then need to explicitly mock the methods using mock.Mock. In addtion, when a return value is needed, this is provided in the call to mock.Mock.

A test is shown below:

def test_handle_button_clicked(self):
    self._presenter.handle_button_clicked()
    self._view.get_value.assert_called_once()

We call the handle_button_clicked function and then use assert_called_once to ensure that the method from the view is called the correct number of times. This is a robust method for checking how many times a function is called.

We could also use self.assertEqual(1, self._view.get_value.call_count) or a python assert statement. However using more specific asserts from the mock and unittest libraries can make intent and error messages clearer.

The last bit of code is to execute the tests:

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