Using mypy means that you can write and run Python in the normal way, gradually adding static type checking over time and checking it outside your program's execution.

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.

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).