Yep there definitely is. Did you do anything in particular with the upload there? Its not the first I've seen it, but haven't a clue the cause.
I'm not sure I understand what you are doing here. The image *may* have provided more information lol.
The use of private is done for this explicit reason. You do not want to allow direct manipulation of the variable by any extending classes. However, it is highly probable that the imported class has a method that allows manipulation (I haven't a clue what this is). I assume this only because you appear to intend to make use of the class itself which in turn needs to take any modifications you perform externally against it. Otherwise, there really is no purpose in linking them together at all.
If you are interrelating the two classes together, perhaps extending the Mechanic class or writing a decorator for it would be warranted? Assuming it has at least a get method on it for the "black" cars, than you can respond accordingly with the "red" ones. I wouldn't recommend hard coding against the number being 50 if it can be avoided (as it sounds right now, if you have 50 its simply 50 - redCars to determine the number of black cars).
Try getting that upload in; it may be easier to see what you are wanting to do with it.