This violates the Liskov Substitution Principle.
If we REALLY had a time machine, then dict would subclass frozendict, and we’d be all set. But to what extent do we need to support ALL the ways to check for an interface? Personally, I think EAFTP is the most “Pythonic”, but if folks want to check up front, then isn’t that what ABCs are for? In which case , we already have Mapping and MutableMapping. So if you have code that checks for Mapping when you need it mutable, then that’s arguably a bug. And if you code checks for dict directly, then it’s arguably unpythonic. That being said, it probably is best not to break working code. Would there be unpleasant consequences to making dict a subclass of FrozenDict ? Or maybe come up with a new name: after all, lists and tuples are independent, even though you *could* think of a tuple as a FrozenList ... -CHB