CODEWITHSUNDEEP
pythondjangounit-testingmocking
Thomas W.
Thomas W.

Reputation: 13

How do I patch a python method for a test case to run?

A project I'm working on uses django for basically everything. When writing a model, I found it necessary to override the save() method to spin off a task to be run by a worker:

class MyModel(models.Model)
    def _start_processing(self):
        my_task.apply_async(args=['arg1', ..., 'argn'])
    def save(self, *args, **kwargs):
        """Saves the model object to the database"""
        # do some stuff
        self._start_processing()
        # do some more stuff
        super(MyModel, self).save(*args, **kwargs)

In my tester, I want to test the parts of the save override that are designated by # do some stuff and # do some more stuff, but don't want to run the task. To do this, I believe I should be using mocking (which I'm very new to).

In my test class, I've set it up to skip the task invocation:

class MyModelTests(TestCase):
    def setUp(self):
        # Mock the _start_processing() method. Ha!
        @patch('my_app.models.MyModel._start_processing')
        def start_processing(self, mock_start_processing):
            print('This is when the task would normally be run, but this is a test!')

        # Create a model to test with
        self.test_object = MyModelFactory()

Since the factory creates and saves an instance of the model, I need to have overwritten the _start_processing() method before that is called. The above doesn't seem to be working (and the task runs and fails). What am I missing?

Upvotes: 1

Views: 107

Answers (1)

Alexandr Tatarinov
Alexandr Tatarinov

Reputation: 4034

First of all, you have to wrap into decorator not the function which you want to use as replacement, but the "scope" in which your mock should work. So, for example, if you need to mock the _start_processing for the whole MyModelTests class, you should place the decorator before the class definition. If only for one test method - wrap only test method with it.

Secondly, define that start_processing function somewhere outside the class, and pass @patch('my_app.models.MyModel._start_processing', new=start_processing), so it will know what to use as a replacement for actual method. But be aware to match the actual method signature, so use just 

def start_processing(self):
    print('This is when the task would normally be run, but this is a test!')

Thirdly, you will have to add mock_start_processing argument to each test case inside this class (test_... methods), just because mocking works like this :).

And finally. You have to be aware about the target you are patching. Your current my_app.models.MyModel._start_processing could be broken. You have to patch the class using the path where it is USED, not where it is DEFINED. So, if you are creating objects with MyModelFactory inside TestCase, and MyModelFactory lives in my_app.factories and it imports MyModel as from .models import MyModel, you will have to use @patch('my_app.factories.MyModel._start_processing'), not 'my_app.models.MyModel._start_processing'.

Hopefully, it helps.

Upvotes: 1

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