The pip3 reflects that I'm using Python 3, rather than Python 2. And the -U option indicates that I'd like to upgrade my installation of mypy, if the package has been updated since I last installed it ...

Python comes with a collection of built-in data types that make common data-wrangling operations easy. Among them is the list, a simple but versatile collection type.

Learn how to use Python’s optional type-hinting syntax to create cleaner and more useful code.