feat: Allow casting List<UInt8> to Binary

crates/polars-compute/src/cast/mod.rs

@@ -8,6 +8,7 @@ mod dictionary_to;
 mod primitive_to;
 mod utf8_to;
 
+use arrow::bitmap::MutableBitmap;
 pub use binary_to::*;
 #[cfg(feature = "dtype-decimal")]
 pub use binview_to::binview_to_decimal;
@@ -237,6 +238,125 @@ pub(super) fn cast_list_to_fixed_size_list<O: Offset>(
     .map_err(|_| polars_err!(ComputeError: "not all elements have the specified width {size}"))
 }
 
+fn cast_list_uint8_to_binary<O: Offset>(list: &ListArray<O>) -> PolarsResult<BinaryViewArray> {
+    let mut views = Vec::with_capacity(list.len());
+    let mut result_validity = MutableBitmap::from_len_set(list.len());
+
+    let u8array: &PrimitiveArray<u8> = list.values().as_any().downcast_ref().unwrap();
+    let slice = u8array.values().as_slice();
+    let mut cloned_buffers = vec![u8array.values().clone()];
+    let mut buf_index = 0;
+    let mut previous_buf_lengths = 0;
+    let validity = list.validity();
+    let internal_validity = list.values().validity();
+    let offsets = list.offsets();
+
+    let mut all_views_inline = true;
+
+    // In a View for BinaryViewArray, both length and offset are u32.
+    #[cfg(not(test))]
+    const MAX_BUF_SIZE: usize = u32::MAX as usize;
+
+    // This allows us to test some invariants without using 4GB of RAM; see mod
+    // tests below.
+    #[cfg(test)]
+    const MAX_BUF_SIZE: usize = 3;
+
+    for index in 0..list.len() {
+        // Check if there's a null instead of a list:
+        if let Some(validity) = validity {
+            // SAFETY: We are generating indexes limited to < list.len().
+            debug_assert!(index < validity.len());
+            if unsafe { !validity.get_bit_unchecked(index) } {
+                debug_assert!(index < result_validity.len());
+                unsafe {
+                    result_validity.set_unchecked(index, false);
+                }
+                views.push(View::default());
+                continue;
+            }
+        }
+
+        // SAFETY: We are generating indexes limited to < list.len().
+        debug_assert!(index < offsets.len());
+        let (start, end) = unsafe { offsets.start_end_unchecked(index) };
+        let length = end - start;
+        polars_ensure!(
+            length <= MAX_BUF_SIZE,
+            InvalidOperation: "when casting to BinaryView, list lengths must fit in u32"
+        );
+
+        // Check if the list contains nulls:
+        if let Some(internal_validity) = internal_validity {
+            if internal_validity.null_count_range(start, length) > 0 {
+                debug_assert!(index < result_validity.len());
+                unsafe {
+                    result_validity.set_unchecked(index, false);
+                }
+                views.push(View::default());
+                continue;
+            }
+        }
+
+        if start - previous_buf_lengths > MAX_BUF_SIZE {
+            // View offsets must fit in u32 (or smaller value when running Rust
+            // tests).
+            buf_index += 1;
+            let (previous, next) = cloned_buffers
+                .last()
+                .unwrap()
+                .split_at(start - previous_buf_lengths);
+            previous_buf_lengths += previous.len();
+            *(cloned_buffers.last_mut().unwrap()) = previous;
+            cloned_buffers.push(next);
+        }
+        let view = View::new_from_bytes(
+            &slice[start..end],
+            buf_index,
+            (start - previous_buf_lengths) as u32,
+        );
+        if !view.is_inline() {
+            all_views_inline = false;
+        }
+        debug_assert_eq!(
+            unsafe { view.get_slice_unchecked(&cloned_buffers) },
+            &slice[start..end]
+        );
+        views.push(view);
+    }
+
+    // Optimization: don't actually need buffers if Views are all inline.
+    if all_views_inline {
+        cloned_buffers.clear();
+    }
+
+    let result_buffers = cloned_buffers.into_boxed_slice().into();
+    let result = if cfg!(debug_assertions) {
+        // A safer wrapper around new_unchecked_unknown_md; it shouldn't ever
+        // fail in practice.
+        BinaryViewArrayGeneric::try_new(
+            ArrowDataType::BinaryView,
+            views.into(),
+            result_buffers,
+            result_validity.into(),
+        )?
+    } else {
+        unsafe {
+            BinaryViewArrayGeneric::new_unchecked_unknown_md(
+                ArrowDataType::BinaryView,
+                views.into(),
+                result_buffers,
+                result_validity.into(),
+                // We could compute this ourselves, but we want to make this code
+                // match debug_assertions path as much as possible.
+                None,
+            )
+        }
+    };
+
+    Ok(result)
+}
+
 pub fn cast_default(array: &dyn Array, to_type: &ArrowDataType) -> PolarsResult<Box<dyn Array>> {
     cast(array, to_type, Default::default())
 }
@@ -258,6 +378,7 @@ pub fn cast_unchecked(array: &dyn Array, to_type: &ArrowDataType) -> PolarsResul
 /// * Fixed Size List to List: the underlying data type is cast
 /// * List to Fixed Size List: the offsets are checked for valid order, then the
 ///   underlying type is cast.
+/// * List of UInt8 to Binary: the list of integers becomes binary data, nulls in the list means it becomes a null
 /// * Struct to Struct: the underlying fields are cast.
 /// * PrimitiveArray to List: a list array with 1 value per slot is created
 /// * Date32 and Date64: precision lost when going to higher interval
@@ -267,7 +388,7 @@ pub fn cast_unchecked(array: &dyn Array, to_type: &ArrowDataType) -> PolarsResul
 ///
 /// Unsupported Casts
 /// * non-`StructArray` to `StructArray` or `StructArray` to non-`StructArray`
-/// * List to primitive
+/// * List to primitive (other than UInt8)
 /// * Utf8 to boolean
 /// * Interval and duration
 pub fn cast(
@@ -326,6 +447,14 @@ pub fn cast(
             options,
         )
         .map(|x| x.boxed()),
+        (List(field), BinaryView) if matches!(field.dtype(), UInt8) => {
+            cast_list_uint8_to_binary::<i32>(array.as_any().downcast_ref().unwrap())
+                .map(|arr| arr.boxed())
+        },
+        (LargeList(field), BinaryView) if matches!(field.dtype(), UInt8) => {
+            cast_list_uint8_to_binary::<i64>(array.as_any().downcast_ref().unwrap())
+                .map(|arr| arr.boxed())
+        },
         (BinaryView, _) => match to_type {
             Utf8View => array
                 .as_any()
@@ -853,3 +982,80 @@ fn from_to_binview(
     };
     Ok(binview)
 }
+
+#[cfg(test)]
+mod tests {
+    use arrow::offset::OffsetsBuffer;
+    use polars_error::PolarsError;
+
+    use super::*;
+
+    /// When cfg(test), offsets for ``View``s generated by
+    /// cast_list_uint8_to_binary() are limited to max value of 3, so buffers
+    /// need to be split aggressively.
+    #[test]
+    fn cast_list_uint8_to_binary_across_buffer_max_size() {
+        let dtype =
+            ArrowDataType::List(Box::new(Field::new("".into(), ArrowDataType::UInt8, true)));
+        let list_u8 = cast(
+            // Values become individual lists:
+            &PrimitiveArray::from_slice([1u8, 2, 3, 4, 5, 6, 7]),
+            &dtype,
+            CastOptionsImpl {
+                wrapped: false,
+                partial: true,
+            },
+        )
+        .unwrap();
+
+        let binary = cast(
+            list_u8.as_ref(),
+            &ArrowDataType::BinaryView,
+            CastOptionsImpl::default(),
+        )
+        .unwrap();
+        let binary_array: &BinaryViewArray = binary.as_ref().as_any().downcast_ref().unwrap();
+        assert_eq!(
+            binary_array.values_iter().collect::<Vec<&[u8]>>(),
+            vec![&[1], &[2], &[3], &[4], &[5], &[6], &[7]]
+        );
+        // max offset of 3 so we need to split:
+        assert_eq!(
+            binary_array
+                .data_buffers()
+                .iter()
+                .map(|buf| buf.len())
+                .collect::<Vec<_>>(),
+            vec![4, 3]
+        );
+    }
+
+    /// Arrow spec requires views to fit in a single buffer. When cfg(test),
+    /// buffers generated by cast_list_uint8_to_binary are of size 3 or smaller,
+    /// so a list of size 4 should cause an error.
+    #[test]
+    fn cast_list_uint8_to_binary_errors_too_large_list() {
+        let values = PrimitiveArray::from_slice([1u8, 2, 3, 4]);
+        let dtype =
+            ArrowDataType::List(Box::new(Field::new("".into(), ArrowDataType::UInt8, true)));
+        let list_u8 = ListArray::new(
+            dtype,
+            OffsetsBuffer::one_with_length(4),
+            values.boxed(),
+            None,
+        );
+
+        let err = cast(
+            &list_u8,
+            &ArrowDataType::BinaryView,
+            CastOptionsImpl::default(),
+        )
+        .unwrap_err();
+        if let PolarsError::InvalidOperation(msg) = err {
+            assert_eq!(
+                msg.as_ref(),
+                "when casting to BinaryView, list lengths must fit in u32"
+            );
+        }
+    }
+}

crates/polars-core/src/chunked_array/cast.rs

@@ -513,6 +513,25 @@ impl ChunkCast for ListChunked {
                     ))
                 }
             },
+            #[cfg(feature = "dtype-u8")]
+            Binary => {
+                polars_ensure!(
+                    matches!(self.inner_dtype(), UInt8),
+                    InvalidOperation: "cannot cast List type (inner: '{:?}', to: '{:?}')",
+                    self.inner_dtype(),
+                    dtype,
+                );
+                let chunks = cast_chunks(self.chunks(), &DataType::Binary, options)?;
+
+                // SAFETY: we just cast so the dtype matches.
+                unsafe {
+                    Ok(Series::from_chunks_and_dtype_unchecked(
+                        self.name().clone(),
+                        chunks,
+                        &DataType::Binary,
+                    ))
+                }
+            },
             _ => {
                 polars_bail!(
                     InvalidOperation: "cannot cast List type (inner: '{:?}', to: '{:?}')",

py-polars/tests/unit/operations/test_cast.py

@@ -656,6 +656,54 @@ def test_invalid_inner_type_cast_list() -> None:
         s.cast(pl.List(pl.Categorical))
 
 
+@pytest.mark.parametrize(
+    ("values", "result"),
+    [
+        ([[]], [b""]),
+        ([[1, 2], [3, 4]], [b"\x01\x02", b"\x03\x04"]),
+        ([[1, 2], None, [3, 4]], [b"\x01\x02", None, b"\x03\x04"]),
+        (
+            [None, [111, 110, 101], [12, None], [116, 119, 111], list(range(256))],
+            [
+                None,
+                b"one",
+                # A list with a null in it gets turned into a null:
+                None,
+                b"two",
+                bytes(i for i in range(256)),
+            ],
+        ),
+    ],
+)
+def test_list_uint8_to_bytes(
+    values: list[list[int | None] | None], result: list[bytes | None]
+) -> None:
+    s = pl.Series(
+        values,
+        dtype=pl.List(pl.UInt8()),
+    )
+    assert s.cast(pl.Binary(), strict=False).to_list() == result
+
+
+def test_list_uint8_to_bytes_strict() -> None:
+    series = pl.Series(
+        [[1, 2], [3, 4]],
+        dtype=pl.List(pl.UInt8()),
+    )
+    assert series.cast(pl.Binary(), strict=True).to_list() == [b"\x01\x02", b"\x03\x04"]
+
+    series = pl.Series(
+        "mycol",
+        [[1, 2], [3, None]],
+        dtype=pl.List(pl.UInt8()),
+    )
+    with pytest.raises(
+        InvalidOperationError,
+        match="conversion from `list\\[u8\\]` to `binary` failed in column 'mycol' for 1 out of 2 values: \\[\\[3, null\\]\\]",
+    ):
+        series.cast(pl.Binary(), strict=True)
+
+
 def test_all_null_cast_5826() -> None:
     df = pl.DataFrame(data=[pl.Series("a", [None], dtype=pl.String)])
     out = df.with_columns(pl.col("a").cast(pl.Boolean))