Pyright type error in transpiled Json.fs: `getCases` returns `Any | list[Unknown]` instead of `Array[str]`

[dbrattli]

Summary

When src/stdlib/Json.fs is transpiled to Python and consumed by a project that runs pyright in standard mode, a type error is reported in the generated json.py:

json.py:29:37 - error: Type "Any | list[Unknown]" is not assignable to declared type "Array"
  "list[Unknown]" is not assignable to "FSharpArray[str]" (reportAssignmentType)

The offending generated line:

cases: Array[str] = getattr(type(o), 'cases', lambda: [])()

This surfaced when bumping Fable.Python from 4.24.0 to 5.0.0 in the Fable repo's Python test project (fable-compiler/Fable#4571 — pyright CI job).

Root cause

In src/stdlib/Json.fs (around line 90):

[<Emit("getattr(type($0), 'cases', lambda: [])()")>]
let private getCases (o: obj) : string array = nativeOnly

The F# return type string array becomes Array[str] (FSharpArray) in the generated Python annotation. But the emit's default branch (lambda: []) returns a plain Python list, not an FSharpArray — so pyright infers the expression's type as Any | list[Unknown], which doesn't satisfy Array[str].

The Fable.Python.Json module is new in v5.0.0 (introduced by #175, expanded in #182 and #184), so this error didn't exist on 4.24.0.

Reproduction

1. In a project referencing Fable.Python 5.0.0, transpile any F# file that pulls in Fable.Python.Json.
2. Run pyright (standard mode, default settings) over the resulting fable_modules/fable_python/stdlib/json.py.
3. Observe the error above on line 29.

Suggested directions

getCases is only used inside fableDefault to read cases.Length and cases.[tag], so the implementation has flexibility. Some options:

1. Replace the Emit with plain F# using the existing private hasattr / getattr helpers to check for cases on type(o) and call it — returning [||] when absent. This keeps the F# type honest and lets Fable generate well-typed Python.
2. Change the return type to something the emit actually produces (e.g. a seq) and adapt call sites.
3. Inline the lookup at the call site so no separate helper is needed.

# type: ignore / cast(Any, ...) / excluding the file are not great options — they hide a real type mismatch.

Workaround for downstream users

Add **/fable_modules/fable_python/stdlib/json.py to the exclude list in the consuming project's pyrightconfig.json until a fix is released.