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A “frozen” dictionary for Python

(lwn.net)
191 points jwilk | 4 comments | 11 Dec 25 09:51 UTC | HN request time: 0.822s | source
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codethief ◴[11 Dec 25 11:17 UTC] No.46230017[source]▶
Next step: Auto-inferring the correct (most narrow) TypedDict type from a frozendict. (Think `const foo = { … } as const` in TypeScript.) I miss this TS feature in Python on the daily.
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mrweasel ◴[11 Dec 25 12:03 UTC] No.46230354[source]▶
What would that do exactly? Auto-generate a TypedDict class?

Bringing up TypedDict is sort of interesting, because it seems like a frozen dictionary could have been implemented by PEP 705, if typing.ReadOnly was enforced at runtime, and not just a hint to a static type checker.

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dmurray ◴[11 Dec 25 12:15 UTC] No.46230430[source]▶
Auto-generate a TypedDict type while type checking, while doing nothing at runtime.

I expect this is not a very big ask and the various typecheckers will have versions of this soon after release.

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1. mrweasel ◴[11 Dec 25 12:29 UTC] No.46230566[source]▶
Okay so basically just "inline":

  class MyType(TypedDict):
    a: str
    b: int
or infer the type hint in:

  my_typed_dict: dict[str, int] = {"a": 5}
It should probably be something like:

  auto_typed: dict[typing.Const, typing.Const] = {"a": 5}
where typing.Const defaults to Any for an empty dict.
replies(1): >>46232616 #
2. codethief ◴[11 Dec 25 15:31 UTC] No.46232616[source]▶
Not sure we're talking about the same thing. Inline TypedDicts are already in the process of being formalized, see https://peps.python.org/pep-0764/ , and have experimental support in e.g. Pyright.

What I meant was that

  foo = {"a": 5}
should be inferred as

  foo: TypedDict[{ "a": Literal[5] }] = {"a": 5}
replies(1): >>46236817 #
3. mrweasel ◴[11 Dec 25 20:38 UTC] No.46236817[source]▶
Ah, I didn't know about PEP 795. I don't really know if I like it though. It looks like something someone invented because they don't want to write classes or specifically want to be able to access a data structure like a dict, for whatever reason.

It basically provides data types, but also ensures that you have no easy way to reuse them, and it will miss cases where two data structures happen to have the same signature.

It's getting a little messy when we have class, namedtuple and TypedDict, which all can do much the same thing.

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4. codethief ◴[12 Dec 25 22:45 UTC] No.46249961{3}[source]▶
> it will miss cases where two data structures happen to have the same signature.

Huh. That's exactly the point: TypedDicts are structural types.

> we have class, namedtuple and TypedDict, which all can do much the same thing.

No, they can't. The former two are nominal types.