How to create an instance of mocked class within another class

Question

Class A has an attribute of another class B.

Class A():
   def __init__(self, b):
      self.b = b

   def get_b_secret(self, job_id):
      x, y = self.b.get_secret(job_id)
      return x, y

Class B():
   def __init__(self, key, url):
      self.key = key
      self.url = url
   def get_secret(job_id):
      # logic to get secret
      # return a tuple
      return x, y

I want to write a unit test for method get_b_secret of class A by mocking B class as a whole.

@patch('b.B')
def test_get_b_secret(self, mock_b):
    mock_b.b.get_secret.return_value = ('x', 'y')
    obj = A(mock_b) 
    expected = ('x','y')
    self.assertEqual(obj.get_b_secret('001'), expected)

I realized that by mocking class B, I am not really instantialzing B to an instance inside of A's instance. That's why when I debug the test, A 's get_b_secret is returning a MagicMock object instead.

I found this article about mocking object. But in that example, the outer class's init doesn't have inner Class object as an argument. So it is a little different. What is the best way to do it?

Answer 1

If you example code is correct, than you don't need to mock class B. You just need to pass mock with function get_secret into class A when you initializing it.

mock = MagicMock()    

mock.return_value.get_secret.return_value = ('x', 'y')
obj = A(mock)
....