For some time now, Python developers have been able to “annotate” names with type information. With Python 3.5, type hints officially became part of the language (see PEP 484 ).

But annotating four million lines of Python code in this way isn’t a straightforward task, here are some of the lesser-known benefits and how Dropbox accomplished the task.

Python’s typing module, used to annotate code with type information, lets you describe the types of a callable (e.g., a function). But that type information can’t be propagated across callables.