I need to create a fake helper class to be used in unit tests (injected into tested classes). Is there some way to use TestCase assertions in such class?
You can use Pytest or Nosetest. Though I'don't know if they have 'assertNotNull' function. I know they can use 'assert' simply for assertion. Or you can use something like assertpy or ptest, if you like, you can search for them on github.