How to assert that a mocked object got assigned a new value

I have a test set up as this:

Suppose I have a class like this, where in the middle I may or may not update a class variable:

  class Supplier():
    cache_dict = {}

    @classmethod
    def get_available_suppliers(cls):
      ...
      if cls.cache_dict.get('time') < end_date:
        cls.cache_dict = cls._fetch_new_cache(...)
      ...
      return available_suppliers

In my test I want to assert that variable did get or did not get updated. How should I go about testing that? Here's what I have so far, but doesn't seem to work.

def test_cache_refreshed(self):
  self.mock_cache_dict = mock.patch('Suppliers.cache_list' new_callable= mock.PropertyMock).start()
  self.mock_cache_dict.return_value =  {'time': '2018/01/01' ...}
  Config.get_available_suppliers()
  #line below does not work, still have the same return_value as before
  self.assertEqual(self.mock_cache_dict.return_value, {'time': '2018/01/08' ...})
  #this way does not work either, as it's comparing PropertyMock to a datetime.
  self.assertEqual(self.mock_cache_dict, {'time': '2018/01/08'...})

Please let me know how I should do this.