Comment 8 for bug 530325

"if not track" could catch any case where track evaluates to False, so None, 0, False, any empty container, etc. would all match this case. "if track is not None" explicitly checks it against None, so 0, False, etc. won't match. While not a big deal for this situation, it's a good habit to use as it makes your code more robust. Additionally, it also hints to people reading your code that you _expect_ track to be None sometimes, which is useful information.