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 ...

The Python array is designed to emulate the behavior of an array in C, and it’s meant chiefly to allow Python to work with C arrays. The array type is useful in those cases, but in almost every ...

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).