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Comment by insanitybit | Hacker News Reader

You can't even represent `Json` in Python's type system because it would require a recursive type.

edit: I think this is actually a Python type annotation limitation but it's possible that's mypy, although I think in 99% of cases those are fine to conflate (I have used other Python type systems).

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In VSCode (which uses PyRight/PyLance) and Python 3.10:

  JSONObject = None | str | int | bool | list["JSONObject"] | dict[str, "JSONObject"]
  # this type checks
  a: JSONObject = {"a": [1, 2, "7", True, {"false": None}]}
  # this doesn't type check
  b: JSONObject = {"a": [1, 2, "7", True, {"false": object()}]}

Cool, guess they finally fixed this. Must've been in the last ~1 year, give or take. Of course, it relies on quoting your types, which is... a matter of taste, I suppose.

You can also use from __future__ import annotations so the quotes become unnecessary. https://peps.python.org/pep-0563/

Does that work with recursive types? I have had mixed results with `from __future__ import annotations` personally, but I haven't written much Python in ~a year or so.

It's worked for many years, but you won't often see it used outside of class definitions because all of the other tools struggle with it (Pylint, Flake8, Pylance, etc. spit out some variation of an undefined variable error).

I don't think it has worked for years. I had used that same exact __annotations__ and my recollection was that recursive types still did not work.

It's worked for years. I've been using it the whole time.

Quoting types that are defined later is a wart but it's not very bad. VSCode's UI will happily handle it as if the quotes weren't there.

You can.

   from typing import Dict, List
   JSON = Dict[str, 'JSON'] | List['JSON'] | float | int | str | None

I would say the only thing ugly here is the string 'JSON' that is used for recursion. From a static safety perspective with an external type checker the static safety effect is identical.

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