Peter Bengtsson: In Python, you have to specify the type and not rely on inference

Unlike TypeScript, if you give a variable a default, which has a type, that variable is implied to always have the type of the default. That's not the case with mypy and ty.

http://www.peterbe.com/plog/in-python-you-have-to-specify-the-type-and-not-rely-on-inference