LGTM, thanks for the cleanup! In the future we might even consider putting all the book_*.py files into a directory / package. That should make it more natural to import them.
https://codereview.appspot.com/583570043/diff/545630043/python/book_base_test.py File python/book_base_test.py (right): https://codereview.appspot.com/583570043/diff/545630043/python/book_base_test.py#newcode48 python/book_base_test.py:48: self.assertEqual(chunks[-2].line_number, 18) I'd start counting from the beginning (ie, chunks[7]): When extending the test in the future, we don't have to recount from the end. https://codereview.appspot.com/583570043/
