Python mock failing call assertion

Question

I have been reading into python mocking but can't get my head around why the following code is failing.

I have two classes, a Potato and a PotatoBag like the following. Figure is stored in food.py and Report is stored in bag.py.

class Potato:
    def create_potato(self):
        pass

    def output_potato(self):
        pass


class PotatoBag:
    def __init__(self, potatoes):
        self.potatoes = potatoes

    def output_to_file(self):
        for fig in self.potatoes:
            fig.create_potato()
            fig.output_potato()

Currently I am trying to unit test the output method so that Report correctly calls create_figure and output_figure from Figure using a mock. This is my test code:

from unittest.mock import MagicMock, patch
from bag import PotatoBag
from food import Potato
import pytest

@pytest.fixture(scope='module')
def potatoes():
    x = Potato()
    y = Potato()
    return [x, y]

@patch('food.Potato')
def test_output_to_file(mock_potato, potatoes):

    test_potato_bag = PotatoBag(potatoes)
    test_potato_bag.output_to_file()

    mock_potato.return_value.create_potato.assert_called()
    mock_potato.return_value.output_potato.assert_called()

Immediately pytest yields an AssertionError stating that create_figure was never called.

_mock_self = <MagicMock name='Potato().create_potato' id='140480853451272'>

    def assert_called(_mock_self):
        """assert that the mock was called at least once
            """
        self = _mock_self
        if self.call_count == 0:
            msg = ("Expected '%s' to have been called." %
                   self._mock_name or 'mock')
>           raise AssertionError(msg)
E           AssertionError: Expected 'create_potato' to have been called.

/home/anaconda3/lib/python3.7/unittest/mock.py:792: AssertionError

What is wrong with my code?

Answer 1

You are passing the Report a list of Figures from your fixture instead of a mock.

Changing your test to...

@patch('figure.Figure')
def test_output_to_file(mock_figure, figures):

    test_report = Report([mock_figure])
    test_report.output_to_file()

    mock_figure.create_figure.assert_called_once()
    mock_figure.output_figure.assert_called_once()

This resolves testing that output_to_file correctly is calling the functions on Figure without actually worrying about setting up a figure and dealing with any side effects or additional complexities that may come with calling those functions. The worries of that can be saved for the unit tests for Figure ;)