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Comment by djinnish | Hacker News Reader

I think astral and meta were both working on their own type-checkers independently. My current understanding is that meta released so they could preempt the initial release of ty. It seems like they're a bit further ahead in development. Not sure if there are going to be any real differences between the two down the line.

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I think they've mentioned earlier that it's really just because PyCon is this week (so a good time to announce new Python tooling).

Sounds a lot like TypeScript and Flow.

Sure, but in this case they are both implementations of a spec defined by PEPs, so a bit more like gcc vs clang (less tightly bound than those, of course, in design decisions). Neither company is trying to invent a new language here.

The current major type checkers, mypy and pyright, are also based on the same PEPs, but you can still see differences between them. For example, my codebase passes pyright in strict mode, but mypy results in a bunch of type errors. I'd expect pyrefly and ty to be slightly different from each other.