Using mock patch to mock an instance method

Question

I'm trying to mock something while testing a Django app using the imaginatively named Mock testing library. I can't seem to quite get it to work, I'm trying to do this:

models.py

from somelib import FooClass

class Promotion(models.Model):
    foo = models.ForeignKey(FooClass)
    def bar(self):
       print "Do something I don't want!"


test.py

class ViewsDoSomething(TestCase):
    view = 'my_app.views.do_something'

    def test_enter_promotion(self):
        @patch.object(my_app.models.FooClass, 'bar')
        def fake_bar(self, mock_my_method):
            print "Do something I want!"
            return True

        self.client.get(reverse(view))

What am I doing wrong?

```bar``` is in fact an "instance method," and its only parameter is ```self```. To be a class method, it would need to be parameterized with ```cls```, and would be callable as ```Promotion.foo()```. — cavaunpeu, Sep 10 '15 at 19:57
Patchable object needs to be quoted like this: `@patch.object('my_app.models.FooClass', 'bar')` — Lasma, Aug 29 '16 at 22:26
@cavaunpeu - not (just) parameterized with `cls`, but more importantly (since `self` and `cls` don't mean anything special in python), decorated with `@classmethod` — dwanderson, Jan 23 '18 at 21:28

score 94 · Accepted Answer · edited Dec 16 '21 at 12:19

94

To add onto Kit's answer, specifying a 3rd argument to patch.object() allows the mocked object/method to be specified. Otherwise, a default MagicMock object is used.

    def fake_bar(self):
        print "Do something I want!"
        return True

    @patch.object(my_app.models.FooClass, 'bar', fake_bar)
    def test_enter_promotion(self):
        self.client.get(reverse(view))
        # Do something I want!

Note that, if you specify the mocking object, then the default MagicMock() is no longer passed into the patched object -- e.g. no longer:

def test_enter_promotion(self, mock_method):

but instead:

def test_enter_promotion(self):

https://docs.python.org/3/library/unittest.mock.html#patch-object

edited Dec 16 '21 at 12:19

bravmi

505
2
7
13

answered Dec 30 '15 at 17:56

storm_m2138

2,281
2
20
18

1

I prefer this implementation. It is more clear, especially for beginners in Unit Testing. – Dorcioman Mar 07 '20 at 17:05
What is the self parameter in this case? Where does it come from? – Kesty Aug 24 '21 at 09:56
@Kesty this must be a method in a Test class. https://docs.python.org/3/library/unittest.html#basic-example – magM Apr 08 '22 at 08:49

score 44 · Answer 2 · answered Dec 12 '11 at 03:40

44

Ah I was confused on where to apply that patch decorator. Fixed:

class ViewsDoSomething(TestCase):
    view = 'my_app.views.do_something'

    @patch.object(my_app.models.FooClass, 'bar')
    def test_enter_promotion(self, mock_method):
        self.client.get(reverse(view))

answered Dec 12 '11 at 03:40

Kit Sunde

35,972
25
125
179

26

Where do you make the connection between the method to mock and the fake implementation now? – physicalattraction Sep 05 '14 at 08:34
@physicalattraction, the connection is made by the argument `mock_method` passed to the test function. I have been able to use this technique in one of my tests. This is useful when you only want to verify if the mocked method was called. – Kalyan Vedala Aug 10 '18 at 15:36
@rcode74: How to patch a method of an (some other object) instance inside the testing method. eg: def my_method_to_be_tested(...): r = some_script.some_class(...); r.how_to_patch_this_method. – Mr. Unnormalized Posterior Sep 10 '18 at 17:33
1

@imsrgadich, you would do something like r.how_to_patch_this_method = MagicMock(). You can check MagicMock documentation to see how to assign behavior to the mock object. – Kalyan Vedala Sep 10 '18 at 19:20

score 6 · Answer 3 · answered Dec 15 '21 at 12:29

If you'd like to do assert_called etc. against the mocked method, use the patch.object and wrap replacement method in a MagicMock(side_effect=), ie:

with patch.object(class_to_mock, attribute_name, \
 MagicMock(side_effect=replacement_method)) as replacement_method_mock:

eg.:

from unittest.mock import patch, MagicMock

def fake_bar(self):
    print "Do something I want!"
    return True

def test_enter_promotion(self):
    with patch.object(my_app.models.FooClass, 'bar', MagicMock(side_effect=fake_bar)) as fake_bar_mock:
        self.client.get(reverse(view))
        # Do something I want!
        fake_bar_mock.assert_called()

Using mock patch to mock an instance method

3 Answers3

Linked

Related