Over the past year and a half, Dropbox has been investing in the development of mypy, a static type checker for Python, as a way to make our multimillion-line Python codebase easier to understand, navigate, and maintain. In this talk, we will discuss the benefits of type annotations, explain how to use them, and give a peak into how mypy works behind the scenes. Mypy is an open-source type-checker for Python which supports the PEP 484 standard for gradual typing. Originally created by Jukka Lehtosalo as part of his PhD thesis in 2013, it is now under active development by a small team at Dropbox which includes David Fisher, Greg Price, and Guido van Rossum. It supports Python 3.2 and higher, as well as Python 2.7 (via type comments).

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