How to supply a mock class method for python unit test?

Question

Let's say I have a class like this.

class SomeProductionProcess(CustomCachedSingleTon):          @classmethod     def loaddata(cls):         """         Uses an iterator over a large file in Production for the Data pipeline.         """         pass 

Now at test time I want to change the logic inside the loaddata() method. It would be a simple custom logic that doesn't process large data.

How do we supply custom implementation of loaddata() at testtime using Python Mock UnitTest framework?

Answer 1

Here is a simple way to do it using mock

import mock   def new_loaddata(cls, *args, **kwargs):     # Your custom testing override     return 1   def test_SomeProductionProcess():     with mock.patch.object(SomeProductionProcess, 'loaddata', new=new_loaddata):         obj = SomeProductionProcess()         obj.loaddata()  # This will call your mock method 

I'd recommend using pytest instead of the unittest module if you're able. It makes your test code a lot cleaner and reduces a lot of the boilerplate you get with unittest.TestCase-style tests.

Answer 2

To easily mock out a class method with a structured return_value, can use unittest.mock.Mock.

from unittest.mock import Mock  mockObject = SomeProductionProcess mockObject.loaddata = Mock(return_value=True) 

EDIT:

Since you want to mock out the method with a custom implementation, you could just create a custom mock method object and swap out the original method at testing runtime.

def custom_method(*args, **kwargs):     # do custom implementation  SomeProductionProcess.loaddata = custom_method