Support shift over grouped windows in cudf-polars

[rjzamora]

Adds cudf-polars support for grouped-window shift, including shift(...).over(...) and shift(..., fill_value=...).over(...).

Partially addresses example/feature (7) in #22032. Like existing cum_sum support, implicit-order shift().over(...) without order_by still falls back for multi-rank execution. I plan to remove that limitation for both cum_sum and shift in a follow-up PR.

[rjzamora]

cc @Matt711 (since you originally implemented over() support)

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between f583139 and 5cca1a4.

📒 Files selected for processing (1)

• python/cudf_polars/tests/streaming/test_rolling.py

🚧 Files skipped from review as they are similar to previous changes (1)

• python/cudf_polars/tests/streaming/test_rolling.py

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📝 Walkthrough

Summary by CodeRabbit

• New Features
  • Added support for grouped shift and shift_and_fill window expressions, including positive/negative offsets and ordered or unordered grouping with proper fill behavior.
• Bug Fixes
  • Improved window expression handling so shift-style ops are correctly recognized as input-order-sensitive when order_by is missing.
  • Added stricter validation: shift offset and shift_and_fill fill must be literal values; shift(...).over(...) without order_by can raise in multi-rank mode based on fallback settings.
• Tests
  • Expanded GPU and streaming coverage for shift(...).over(...), including missing-position semantics and new multi-rank error scenarios.

Walkthrough

Adds grouped window shift and shift_and_fill support, including evaluation, translation, streaming fallback detection, literal argument validation, and GPU, single-rank, and multi-rank tests.

Changes

Grouped window shift support

Layer / File(s)	Summary
Shift operation contracts python/cudf_polars/cudf_polars/dsl/expressions/rolling.py, python/cudf_polars/cudf_polars/dsl/translate.py, python/cudf_polars/cudf_polars/dsl/utils/aggregations.py	Adds ShiftOp, recognizes shift-family window expressions during decomposition and translation, and validates literal offsets and fill values.
Grouped shift evaluation python/cudf_polars/cudf_polars/dsl/expressions/rolling.py	Evaluates grouped shifts using ordered group scans, per-expression offsets and fills, and restoration of input row order.
Streaming input-order handling python/cudf_polars/cudf_polars/streaming/utils.py, python/cudf_polars/cudf_polars/streaming/select.py	Expands multi-rank detection and fallback handling to input-order-sensitive cum_sum, shift, and shift_and_fill windows without order_by.
Shift behavior validation python/cudf_polars/tests/expressions/test_rolling.py, python/cudf_polars/tests/streaming/test_rolling.py, python/cudf_polars/tests/streaming/test_spmd.py	Adds ordered and unordered grouped shift tests, fill-value cases, single-rank execution checks, multi-rank results, and expected unsupported-operation errors.

Estimated code review effort: 4 (Complex) | ~45 minutes

Possibly related issues

• rapidsai/cudf#22032 — Directly tracks support for shifted over() expressions implemented by this PR.

Possibly related PRs

• rapidsai/cudf#23206 — Provides related window-expression decomposition and streaming fallback handling generalized here for shift operations.

Suggested labels: improvement

Suggested reviewers: nirandaperera

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 16.67% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: adding grouped-window shift support in cudf-polars.
Description check	✅ Passed	The description accurately matches the grouped-window shift support and the remaining multi-rank fallback limitation.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[Matt711]

Looks good, only comment is can you make sure either we fallback or support a nested expression like pl.col("x").shift(1).fill_null(strategy="forward").over("g")?

[rjzamora]

/merge

[rjzamora]

Update: I just pushed #23466 to enable cum_sum(...).over(...) and shift(...).over(...) without the order_by argument.