python side_effect - mocking behavior of a method

Question

In the mock, I want a certain function to return a new value in the test. This is how i did it.

Class MyClass:

      my_var = None  

      def foo(self, var1):
          return somevalue

      def bar(self):
          my_var = foo(1)

Class TestClass(unittest.TestCase):
      myClass = MyClass() 

      def _side_effect_foo(var1):
           if condition:
                return new_value

      @patch("MyClass", "foo", side_effect='_side_effect_foo')
      def test_foo(self):
           self.myClass.bar()

This gives me a error:

Can't pass kwargs to a mock we aren't creating.

Am I using the correct format of side_effect?

Thanks for any help!

Answer 1

There are a few issues here.

1. The arguments you're passing to the @patch decorator aren't quite right. The target should be specified as a single argument:

   @patch('MyClass.foo')
   

2. Your decorated test function will need to accept an additional argument for the patched target - e.g. mock_foo:

   @patch('MyClass.foo')
   def test_foo(self, mock_foo):
   

3. You can then set the side effect on mock_foo:

   @patch('MyClass.foo')
   def test_foo(self, mock_foo):
       mock_foo.side_effect = self._side_effect_foo
   

4. In addition, _side_effect_foo() should be made a proper instance method with self as the first argument:

   def _side_effect_foo(self, var1):
       if condition:
           return new_value
   

5. Lastly, you should instantiate MyClass either inside the test method itself, or inside the setUp() method:

   def setUp(self):
       self.myClass = MyClass()
   
   @patch('MyClass.foo')
   def test_foo(self, mock_foo):
       mock_foo.side_effect = self._side_effect_foo
       self.myClass.bar()