fix: preserve row count for zero-column DataFrames in Arrow export

DataFrameStreamIterator used first_col_n_chunks() to determine how many
batches to emit. For zero-column DataFrames this returns 0, so the
iterator produced no batches and the resulting Arrow table had 0 rows.

Use max(1, first_col_n_chunks()) so that zero-column frames still emit
one batch. Also store the DataFrame height so that when there are no
columns the batch length comes from the frame height rather than trying
to index into an empty columns vec.

Fixes #26834

Author: root

crates/polars-python/src/interop/arrow/to_py.rs

@@ -120,6 +120,7 @@ pub struct DataFrameStreamIterator {
     dtype: ArrowDataType,
     idx: usize,
     n_chunks: usize,
+    height: usize,
 }
 
 impl DataFrameStreamIterator {
@@ -135,7 +136,8 @@ impl DataFrameStreamIterator {
                 .collect(),
             dtype,
             idx: 0,
-            n_chunks: df.first_col_n_chunks(),
+            n_chunks: usize::max(1, df.first_col_n_chunks()),
+            height: df.height(),
         }
     }
 
@@ -151,17 +153,17 @@ impl Iterator for DataFrameStreamIterator {
         if self.idx >= self.n_chunks {
             None
         } else {
-            // create a batch of the columns with the same chunk no.
-            let batch_cols = self
+            let batch_cols: Vec<ArrayRef> = self
                 .columns
                 .iter()
                 .map(|s| s.to_arrow(self.idx, CompatLevel::newest()))
-                .collect::<Vec<_>>();
+                .collect();
             self.idx += 1;
 
+            let len = batch_cols.first().map_or(self.height, |c| c.len());
             let array = arrow::array::StructArray::new(
                 self.dtype.clone(),
-                batch_cols[0].len(),
+                len,
                 batch_cols,
                 None,
             );

py-polars/tests/unit/interop/test_interop.py

@@ -70,6 +70,17 @@ def test_arrow_empty_dataframe() -> None:
     assert df2.shape == (0, 1)
 
 
+
+def test_arrow_zero_column_dataframe_preserves_rows() -> None:
+    df = pl.DataFrame(height=5)
+    assert df.shape == (5, 0)
+
+    tbl = pa.table(df)
+    assert tbl.shape == (5, 0)
+
+    tbl2 = df.to_arrow()
+    assert tbl2.shape == (5, 0)
+
 def test_arrow_dict_to_polars() -> None:
     pa_dict = pa.DictionaryArray.from_arrays(
         indices=np.array([0, 1, 2, 3, 1, 0, 2, 3, 3, 2]),