Using Python unittest, how can I ensure that a Class was instantiated?

Question

Given the following:

class toTest(object)
    def createObject(self):
        self.created = toCreate()

class toCreate(object):
    def __init__(self):
        pass

# testFile.py
import unittest
class TestThing(unittest.TestCase):
    def testCreated(self):
        creator = toTest()
        toTest.createObject()
        # -- Assert that the 'toCreate' object was in fact instantiated

...how can I ensure that toCreate was in fact created? I have tried the following:

def testCreated(self):
    created = MagicMock(spec=toCreate)
    toTest.createObject()
    created.__init__.assert_called_once_with()

But I get the following error: AttributeError: Mock object has no attribute 'init'. Am I misusing the MagicMock class, and if so how?

JohnAllen · Accepted Answer · 2022-06-27 09:54:12Z

1

unitetest.mock have two main duties:

Define Mock objects: objects that are designed to follow your screenplay and record every access to your mocked object
Patch references and recover the original state

In your example you need both functionalities: Patching toCreate a class reference by a mock where you can have complete behavior control. There are a lot of ways to use patch, some details to take care on how use it and caveats to know.

In your case you should patch toCreate a class instance and check if you call the Mock that patch used to replace the constructor:

class TestThing(unittest.TestCase):
    @patch("module_where_toCreate_is.toCreate")
    def testCreated(self, mock_toCreate):
        creator = toTest()
        toTest.createObject()
        mock_toCreate.assert_called_with()

edited Jun 27, 2022 at 9:54

JohnAllen

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answered Jan 12, 2016 at 20:11

Michele d'Amico

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