To clarify, mypy already has compile-time (well, static checker-time) enforcement of protocols (https://mypy.readthedocs.io/en/stable/protocols.html) and ABCs (https://mypy.readthedocs.io/en/stable/class_basics.html#abst...). Maybe that's what you meant? The first time I read your comment I interpreted as "if there is interest [on the part of core Python developers]".