feat(is_dst): real per-backend implementation (item 65) - #291
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- require timezone at build time (InvalidOptionValueError when omitted) - protocol/backend signatures tightened to timezone: str (no default) - polars: native dt.convert_time_zone + dt.dst_offset() != 0 - narwhals: hand-rolled two-reference-offset algorithm (no native dst_offset equivalent); avoids the narwhals 2.24.0 + pandas 3.0.5 chained-convert_time_zone bug for America/New_York and America/Los_Angeles by never deriving utc_naive by converting back from local_naive - ibis: declared_unsupported (no DST/timezone-offset primitive at all); new is_dst.timezone UNSUPPORTED capability fact on ibis family-default + ibis-duckdb - new cross-backend test file, 11 cases, all passing (7 temporal backends x hemisphere pairs + 3 ibis capability-gate cases) - fixed two pre-existing tests written for the old permissive no-arg/placeholder behavior (test_is_dst_builder_validation, test_remaining_ops_results docstring)
…zone (item 65)
- add IS_DST smoke override (America/New_York UTC pair) to _smoke_helpers.py
so the generic introspection-driven smoke/reachability/signature-conformance
suites synthesize a valid IANA timezone instead of the generic 'x' placeholder
- register is_dst.timezone in the disposition-matrix open-value domains
(option_disposition.py OPTION_DTYPES + _MA_OPTION_VALUE_DOMAINS), mirroring
to_timezone exactly (same value class, same ibis-unsupported shape)
- add a full is_dst OptionCell/OptionProbeRegistration/InvalidOptionRejection
registration section to test_arg_types_datetime.py so the new ibis capability
fact is backed by a declared disposition cell, and every (backend, timezone)
combination gets a runtime probe assertion
- fix test_is_dst AST test and test_is_dst_builder_validation (cross_backend)
to pass the now-required timezone argument
- regenerate docs/reference/expression-coverage.{md,json} + scoped variant
for the new is_dst.timezone ibis capability fact
- ruff --fix: drop unused Optional imports in the four is_dst signature files
Full suite: 22543 passed, 0 failed. ruff clean. mypy: 97 pre-existing
import-untyped/found-twice baseline errors only, no errors in touched code.
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Summary
Replaces the
is_dstplaceholder (return Falseon every backend) with a real per-backend implementation, gated by a capability fact.timezoneis now required —is_dst()with no timezone raisesInvalidOptionValueError.dt.convert_time_zone(tz).dt.dst_offset() != 0.dst_offsetequivalent); verified againstzoneinfoacross 7 zones, both hemispheres, on narwhals-polars and narwhals-pandas. Constructed to avoid a narwhals 2.24.0 + pandas 3.0.5 bug where a secondconvert_time_zonechained onto aAmerica/New_York/America/Los_Angeles-targeted expression raisesAttributeError.declared_unsupported— no DST/timezone-offset primitive exists at all. Enforced via a newis_dst.timezoneCapabilityFact(family default +ibis-duckdb).Spec / Plan
mountainash-central/.../specs/2026-08-15-is-dst-real-implementation-design.md(rev 2, adversarially reviewed + empirically verified against pinned library versions)mountainash-central/.../plans/2026-08-15-is-dst-real-implementation.md(rev 2, 6 tasks)Verification
tests/expressions/cross_backend/test_datetime_is_dst.py: 11 cases (7 temporal backends × hemisphere pairs + 3 ibis capability-gate cases), all passing.ruff check: clean.mypy: only pre-existing baselineimport-untyped/found-twicenoise, no errors in touched code.docs/reference/expression-coverage.{md,json}for the new capability fact.🤖 Generated with SDD (spec → plan → execute)