fix: drop_nulls with subset of length>1

[FBruzzesi]

What type of PR is this? (check all applicable)

• 💾 Refactor
• ✨ Feature
• 🐛 Bug Fix
• 🔧 Optimization
• 📝 Documentation
• ✅ Test
• 🐳 Other

Related issues

• Closes [Bug]: Run into safety assertion on reuse_series_implementation for drop_nulls #1088

Checklist

• Code follows style guide (ruff)
• Tests added
• Documented the changes

If you have comments or can explain your changes, please do so below.

This might not be the permanent fix to the issue, consider it as a hotfix for now

[MarcoGorelli]

thanks for spotting this

i think the issue is just that any_horizontal should always only just return a single column, so we could do

             root_names=combine_root_names(parsed_exprs),
-            output_names=parsed_exprs[0]._output_names,
+            output_names=parsed_exprs[0]._output_names[:1],
         )
 
     def any_horizontal(self, *exprs: IntoPandasLikeExpr) -> PandasLikeExpr:
@@ -262,7 +262,7 @@ class PandasLikeNamespace:
             depth=max(x._depth for x in parsed_exprs) + 1,
             function_name="any_horizontal",
             root_names=combine_root_names(parsed_exprs),
-            output_names=parsed_exprs[0]._output_names,
+            output_names=parsed_exprs[0]._output_names[:1],
         )

and that should fix it without needing to do other changes

[FBruzzesi]

parsed_exprs[0]._output_names can easily be None though.

Do you like:

- output_names=parsed_exprs[0]._output_names,
+ output_names=(parsed_exprs[0]._output_names or [None])[:1],

[MarcoGorelli]

ah you're right - let's make a function then, just like we have for combine_root_names one line above? in there we can explain that we follow the right-hand-rule like Polars does

[FBruzzesi]

right-hand-rule like Polars does

left-most?

let's make a function then

Not sure I follow 100%. combine_root_names seems generic-ish.

For this (horizontal) reduction I would expect to have:

• left-most output name if available
• otherwise? left most root name?

[MarcoGorelli]

yup, left-most, sorry :)

so parsed_exprs[0]._output_names[:1] if parsed_exprs[0]._output_names is not None else None

[MarcoGorelli]

love this

thanks @FBruzzesi !