Gradual typing & mypy
P31.typing.01 · Audience: guest, it-ml, language-pro
Python is dynamically typed, but annotations are gradual: you add them where
they pay, they cost nothing at runtime, and a checker like mypy reads them to
catch bugs before the code runs — this repo gates on mypy packages/compute. In
the browser we can't run mypy, so these rungs grade the annotations by
inspecting them (typing.get_type_hints) — the same information a checker
reads.
ⓘ Concept: Written down, checked, read by everyone
def f(x: int) -> str promises what goes in and what comes out. It has no runtime cost — Python ignores it — but mypy checks it statically, your editor autocompletes from it, and teammates rely on it. The annotation is documentation that cannot drift, because the checker enforces it.Why it matters — A signature you can trust means you don't read the body — and mypy turns a whole class of bugs into a red build instead of a 2am incident.
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🎓 Practice ladder
2 graded rungs · ~20 minAnnotate a function with an Optional return, then narrow an
optional list before using it.
Rung 1 — annotate a function and its Optional return
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Rung 2 — narrow an Optional in a list
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Basic annotations are a habit; the next module adds the machinery that types the hard parts — generics, Protocols, and typed decorators.
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