Dropbox is a heavy user of the mypy type checker, recently passing three million lines of type-annotated Python code, with over half of that added in 2018. Type checking is helping find bugs, making code easier to under stand, enabling refactors, and is an important aid to our ongoing Python 3 migration. In this talk, we discuss how we got there. We’ll talk about what we tried in order to get our engineers to type annotate their code—what worked, what didn’t, and what our engineers had to say …