fix(zarr-metadata): make JSONValue's array arm covariant - #4264
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d-v-b merged 3 commits intoAug 14, 2026
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`list["JSONValue"] | tuple["JSONValue", ...]` is invariant in the element type, so a value typed with any narrower element -- a `list[str]` field on a TypedDict, a `Sequence[float]` -- was not assignable to `JSONValue`, and a TypedDict carrying such fields was not assignable to `Mapping[str, JSONValue]`. pyright's diagnostic for the failure suggests the fix verbatim: "Consider switching from list to Sequence which is covariant." The array arm is now `Sequence["JSONValue"]`. The docstring records the deliberate type-level cost (`Sequence` admits `str`/`bytes`; runtime narrowing must exclude them regardless of the alias's spelling). Found while aliasing zarr-cm's JsonValue to this type: the two aliases are structurally identical except for this arm, and with it changed, pyright unifies them across the package boundary. Assisted-by: ClaudeCode:claude-opus-5
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SMWG (self-merge when green) |
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Consume the pending news fragments — zarr-developers#4232's constant-naming-grammar removal note, zarr-developers#4264's note widening `JSONValue`'s array arm to the covariant `Sequence`, and zarr-developers#4248's sdist allowlist — into CHANGELOG.md via towncrier for the zarr_metadata-v0.5.0 release. Minor, not patch: the `JSONValue` widening changes a published type's meaning for every consumer that annotates against it, and zarr-developers#4232 removes the old version-last constant spellings outright. Assisted-by: ClaudeCode:claude-opus-5
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* chore(zarr-metadata): note the sdist allowlist, and let misc entries speak #4248 gave this package's sdist an explicit allowlist and merged without a news fragment, so a user-visible packaging change was about to miss the 0.5.0 notes. Add one. Filing it as `misc` exposed that towncrier's built-in `misc` type sets `showcontent = false`: the entry would render as a bare PR link, which tells a reader nothing. Restate all five types the `changes/README.md` menu offers — declaring any type replaces the built-in set — as the defaults verbatim except for `misc`, which now shows its content. A change worth a release note is worth a sentence, whatever its category. Assisted-by: ClaudeCode:claude-opus-5 * chore(zarr-metadata): build 0.5.0 changelog Consume the pending news fragments — #4232's constant-naming-grammar removal note, #4264's note widening `JSONValue`'s array arm to the covariant `Sequence`, and #4248's sdist allowlist — into CHANGELOG.md via towncrier for the zarr_metadata-v0.5.0 release. Minor, not patch: the `JSONValue` widening changes a published type's meaning for every consumer that annotates against it, and #4232 removes the old version-last constant spellings outright. Assisted-by: ClaudeCode:claude-opus-5
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I've also hit similar things before, and I think you do want this to be covariant. |
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Summary
list[T] | tuple[T] is invariant but Sequence[T] is covariant. In the context of our JSONValue type, we want covariance, to support intuitive propagation of the "isintance" operator when narrowing JSONValue. That means we need to change our JSONValue union. This is technically a breaking change and so the next release of zarr-metadata will indicate this. Why is it breaking? because strings inhabit Sequence[str], but not list[JSONValue] | tuple[JSONValue]. Routines handling JSONValue-shaped inputs will need to guard for strings where previously they did not. Low blast area but a teeny blast nonetheless.
cc @kylebarron i doubt this affects zarrista but I will do a check in any case.
Written by claude, with an original PR here: d-v-b#295
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TODO