https://docs.python.org/3.6/whatsnew/3.6.html#pep-526-syntax-for-variable-annotations https://www.python.org/dev/peps/pep-0526
https://docs.python.org/3.6/whatsnew/3.6.html#pep-526-syntax-for-variable-annotations
https://www.python.org/dev/peps/pep-0526