Discussion: Runtime error when overriding a mutable key with a `ReadOnly` one

[JanEricNitschke]

Follow up from here: https://github.com/python/typing_extensions/issues/606#issuecomment-3277382801

Background: Currently CPython emits a TypeError when a mutable key from a base TypedDict is overwritten by a ReadOnly one of a child: https://github.com/python/cpython/blob/805e3368d6d07e58430654d1365283924fdf4143/Lib/typing.py#L3151

and it has an explicit test for that behavior: https://github.com/python/cpython/blob/805e3368d6d07e58430654d1365283924fdf4143/Lib/test/test_typing.py#L9077

However typing_extensions permits and tests for that:

• https://github.com/python/typing_extensions/blob/main/src/typing_extensions.py#L1221
• https://github.com/python/typing_extensions/blob/main/src/test_typing_extensions.py#L5000

Originally typing_extensions had the same behaviour as CPython, however that was changed: https://github.com/python/typing_extensions/commit/d6c50f585c386490d38ad6b8ce5543aed6e633a2

This adjustment was not taken into account when the CPython implementation was done: https://github.com/python/cpython/pull/116350

Although this point was later raised in the issue (after the merge of the PR): https://github.com/python/cpython/issues/116127#issuecomment-3176307396

My opinion: From what i understand the TypeError in CPython for this behaviour is pretty unique, as i dont think any other such attribute is runtime enforced.

typing.Final and typing.ReadOnly even specify "There is no runtime checking for this property." in the docs. (Although that obviously refers them only being read, but still).

Additionally the typing docs in general state "The Python runtime does not enforce function and variable type annotations. They can be used by third party tools such as type checkers, IDEs, linters, etc.". Lastly, neither the docs nor the PEP say anything about such a type error existing.

So i definitely feel like the optimal thing here would be to not have a runtime error.

On top of that i feel like removing a TypeError is probably a less intrusive change than adding one, even though changes in CPython are obviously more impactful than ones in typing_extensions.