[Anju522]

Description

PendingDeprecationWarning is hidden by default, so users calling .apply never saw it.

Changed the PendingDeprecationWarning of Dataset.apply, both GroupBy.apply methods (DataArray and Dataset) and both Resample.apply methods to a FutureWarning, so that users can now see the warning.

Tests were already there for checking PendingDeprecationWarning of Dataset.apply, changed that to check for FutureWarning. Added similar tests for GroupBy.apply and Resample.apply ( they check for the warning and whether the result of apply is same as map).
Added this set of changes to the whats-new.rst under v2026.09.1's Deprecations section.

Checklist

• Part of Tracking: open deprecations #11721
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst

[headtr1ck]

LGTM, thanks

[headtr1ck]

Could you use emit_user_level_warning from xarray.core.utils for these? It finds the first stack frame outside xarray, so the warning points at the user's code even if .apply is reached through another xarray call, and it's what the other #11721 PRs use:

emit_user_level_warning(
    "Dataset.apply is deprecated, use Dataset.map instead.",
    FutureWarning,
)

[Anju522]

Got it, I saw that there were only two warnings.warn usage on xarray/core/resample.py , so after replacing it, I removed the import as well.