Earlier quoted context omitted.
> The way these type checkers get fast is usually by not supporting the crazy rich reality of realworld python code. Or in this case, writing it in Rust... mypy is written in Python. People have forgotten that Python is really, really slow for CPU-intensive operations. Python's performance may not matter when you're writing web service code and the bottlenecks are database I/O and network calls, but for a tool that's…
(ty developer here) Currently we default to our oldest supported Python version, in which `datetime.UTC` really doesn't exist! Use `--python-version 3.12` on the CLI, or add a `ty.toml` with e.g. ``` [environment] python-version = "3.12" ``` And we'll find `datetime.UTC`. We've discussed that this is probably the wrong default, and plan to change it.
If I saw something like "datetime.UTC doesn't exist", I'd immediately think "wait, was that datetime.utc", not "ooh it got added in 3.11, I need to change my Python version"