fix: Struct filter by index

[barak1412]

Fixes #18732.

[cmdlineluser]

Thanks @barak1412

I think negative indexing is normally allowed:

>>> df.with_columns(a = pl.col.foo.struct[-1])
# shape: (2, 2)
# ┌───────────┬─────┐
# │ foo       ┆ a   │
# │ ---       ┆ --- │
# │ struct[1] ┆ i64 │
# ╞═══════════╪═════╡
# │ {1}       ┆ 1   │
# │ {2}       ┆ 2   │
# └───────────┴─────┘

The logic for that seems to be here:

polars/crates/polars-plan/src/dsl/function_expr/struct_.rs

Lines 26 to 27 in 6561eba

	FieldByIndex(index) => mapper.try_map_field(|field| {
	let (index, _) = slice_offsets(*index, 0, mapper.get_fields_lens());

[codecov]

Codecov Report

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@@            Coverage Diff             @@
##             main   #18778      +/-   ##
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+ Coverage   79.84%   79.86%   +0.02%     
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  Lines      205576   205637      +61     
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==========================================
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[barak1412]

@cmdlineluser Sure I will look into it.

[barak1412]

@cmdlineluser Fixed

[ritchie46]

I appreciate the fix, but this isn't how it should be fixed. The expression should be replaced by a field by name, hence the panic. We should never access data by index as that doesn't allow us to remove fields we don't need.

[barak1412]

@ritchie46 Sure, I am glad to learn

So the right way is given index, fetch the right field and then use its name to fetch the data in the expression level?

[ritchie46]

So the right way is given index, fetch the right field and then use its name to fetch the data in the expression level?

Yes, but we already have that logic, it seems that we don't hit that in the filter

Note that select works as expected:

df = pl.DataFrame({"foo": [{"a":1},{"a":2}]})
df.select(pl.col.foo.struct[0] == 1)

I think you should look into DSL to IR conversion and look what we do differently in Select vs Filter.

[barak1412]

@ritchie46

Thanks for the guidance, should be the right fix now.