A shorter, possibly easier to understand writeup from six months ago: https://www.mikeash.com/pyblog/friday-qa-2015-11-20-covariance-and-contravariance.html htt

A shorter, possibly easier to understand writeup from six months ago: https://www.mikeash.com/pyblog/friday-qa-2015-11-20-covariance-and-contravariance.html https://www.mikeash.com/pyblog/friday-qa-2015-11-20-covarian... I believe it was written by an HN commenter, and accumulated 67 comments: https://news.ycombinator.com/item?id=10606355 https://news.ycombinator.com/item?id=10606355