If youre doing tdd or writing tests like this you have bigger problems. Type safety is easy in a dynamic language. Stop mutating variables and stop mixing type

If youre doing tdd or writing tests like this you have bigger problems. Type safety is easy in a dynamic language. Stop mutating variables and stop mixing types. Suddenly that whole class of errors evaporates. No need to test things that are impossible. And for those times you want some magic in your life, nothing is stopping you, no need for generics, just proceed with caution and test accordingly.