They were "on the Python Typing Council and helped put together the spec, the conformance test suite, etc" so I assume they are an expert on Python typecheckers
I didn’t write it for parent lol. I guess I should be more careful with “you”.
TypeScript will use flow typing to determine the type as number[] in this code:
const x = []
x.push(1)
type t = typeof x // number[]
TypeScript takes the same approach in this scenario, and I assume this helps both be fast.