I have a Python class structure similar to the following example:

class Foo:
   def start(self):
     # do something

class FooBar(Foo):
   def __init__(self, param):
     self.param = param

   def run(self):
     # do something else

class ProductionClass:
   def use_foobar(self):
     foo = FooBar("string")
     foo.start()

Now I want to write a test in which the start method on FooBar (inherited from Foo) is mocked so that the run method defined directly in FooBar is called instead.

I tried it this way:

def test_something(self):
    self.foobar_patcher = patch.object(FooBar, 'start', side_effect=self.mock_start)
    self.foobar_patcher.start()

    ProductionClass().use_foobar()

def mock_start(self):
    the_production_class_instance.run()

In this example, the_production_class_instance is meant to be the instance of FooBar where the start() method was called on, but I can’t seem to find a way where to get this instance. The instance is created inside ProductionClass().use_foobar() and discarded after use, so there is no real way to access it from the outside.

Is there a way to achieve this with unittest?