patch: group by n_unique
#917
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FBruzzesi
commented
Sep 6, 2024
narwhals/_arrow/group_by.py
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| "std": "stddev", | ||
| "var": "variance", |
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Found these other two cases while reading pyarrow grouped table aggregations (https://arrow.apache.org/docs/python/compute.html#py-grouped-aggrs)
FBruzzesi
commented
Sep 6, 2024
narwhals/_dask/group_by.py
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| def n_unique() -> dd.Aggregation: | ||
| import dask.dataframe as dd # ignore-banned-import | ||
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| return dd.Aggregation( | ||
| name="nunique", | ||
| chunk=lambda s: s.apply(lambda x: list(set(x))), | ||
| agg=lambda s0: s0.obj.groupby(level=list(range(s0.obj.index.nlevels))).sum(), | ||
| finalize=lambda s1: s1.apply(lambda final: len(set(final))), | ||
| ) |
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This comes from the dask documentation itself.
For some reason, nunique kw only works in
df_dd.groupby("a").b.nunique()but not in agg context
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thanks for digging this up in the dask docs!
I think this is way more complex than it needs to be - i've pushed something much simpler which seems to work
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MarcoGorelli
approved these changes
Sep 6, 2024
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thanks @FBruzzesi !
the main issue missing was the handling of null values, but i've added some commits to take care of that
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Related issues
groupby-aggraises #915Checklist
If you have comments or can explain your changes, please do so below.
Attempt for #915, comments in the code