feat: when-then broadcasting#2663
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camriddell merged 14 commits intonarwhals-dev:mainfrom Jun 12, 2025
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Rebased on top of #2662 do not merge until that PR lands in main. |
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thanks @camriddell ! could you rebase again please? |
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narwhals/_dask/namespace.py
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| if self._otherwise_value is None: | ||
| return [then_series.where(condition)] | ||
| otherwise_value = get_dask_expr()._expr.Where._defaults["other"] |
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this is the only part that jumps out to me as suspicious 🤔
MarcoGorelli
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thanks @camriddell ! really solid work here! really impressed you figured out all the details, and I'm excited that we can support nw.when(nw.col('a').count()>0).then(nw.col('a').sum()) now 🥳
I just made two updates:
- not using private methods from Dask. In general, doing that kind of thing gives a very high chance of future failures, i'd advising against doing that whenever possible
- replace a couple of
if x is Truewith justif x(unless there was some reason i missed for usingif x is True?
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This adds broadcasting behavior to the
when/thenpattern similar to Polars (see #2645) for our eager backends and dask.The implementation for the Lazy backends will require #2652 unless we want to introduce some hacky workarounds (e.g. we may be able to do this by joining after an aggregation to "broadcast" results; however I haven't fully thought through how this would look in the codebase).