Guido van Rossum is the author of the Python programming language. It supports Python 3.2 and higher, as well as Python 2.7 (using function signatures in type comments, per a recent update to PEP 484). Mypy supports the PEP 484 standard for gradual typing in Python. It was originally created by one of the speakers, Jukka Lehtosalo, in support of his PhD thesis, before he joined Dropbox. Mypy is an open-source type-checker for Python. We will also describe some of the many improvements we’ve made to mypy in the process, as well as some other tools that come in handy. In this talk we will discuss lessons learned and show how you too can start type-checking your legacy Python 2.7 code, one file at a time. As a first step towards migrating to Python 3, as well as to generally make our code more navigable, we are annotating our code with type annotations using the PEP 484 standard and type-checking the annotated code with mypy. Dropbox has several million lines of production code written in Python 2.7.

Dropbox programming languages