next try

Cargo.lock

@@ -908,7 +908,7 @@ dependencies = [
 
 [[package]]
 name = "qroissant-arrow"
-version = "0.3.0"
+version = "0.3.1"
 dependencies = [
  "arrow-array",
  "arrow-buffer",
@@ -926,7 +926,7 @@ dependencies = [
 
 [[package]]
 name = "qroissant-core"
-version = "0.3.0"
+version = "0.3.1"
 dependencies = [
  "bytemuck",
  "bytes",
@@ -938,11 +938,11 @@ dependencies = [
 
 [[package]]
 name = "qroissant-kernels"
-version = "0.3.0"
+version = "0.3.1"
 
 [[package]]
 name = "qroissant-python"
-version = "0.3.0"
+version = "0.3.1"
 dependencies = [
  "bb8",
  "bytes",
@@ -962,7 +962,7 @@ dependencies = [
 
 [[package]]
 name = "qroissant-transport"
-version = "0.3.0"
+version = "0.3.1"
 dependencies = [
  "bytes",
  "futures",

Cargo.toml

@@ -3,7 +3,7 @@ members = ["crates/qroissant-core", "crates/qroissant-kernels", "crates/qroissan
 resolver = "3"
 
 [workspace.package]
-version = "0.3.0"
+version = "0.3.1"
 edition = "2024"
 license = "Apache-2.0"
 repository = "https://github.com/qroissant/qroissant"

crates/qroissant-arrow/src/projection.rs

@@ -1,6 +1,5 @@
 //! Arrow projection: converts decoded q `Value` trees into Arrow arrays.
 
-use std::mem::size_of;
 use std::ptr::NonNull;
 use std::sync::Arc;
 
@@ -601,9 +600,9 @@ fn project_vector(vector: &Vector, opts: &ProjectionOptions) -> ProjectionResult
                 validity = merge_infinity_i64(src, Q_INF_TIMESTAMP, Q_NINF_TIMESTAMP, validity);
             }
             let nulls = to_null_buffer(validity);
-            let mut bytes_buf = vec![0u8; src.len() * size_of::<i64>()];
-            copy_and_offset_timestamps(src, bytemuck::cast_slice_mut(&mut bytes_buf));
-            let buf = Buffer::from_vec(bytes_buf);
+            let mut typed_buf = vec![0i64; src.len()];
+            copy_and_offset_timestamps(src, &mut typed_buf);
+            let buf = Buffer::from_vec(typed_buf);
             let arr: ArrayRef = Arc::new(TimestampNanosecondArray::new(
                 ScalarBuffer::new(buf, 0, src.len()),
                 nulls,
@@ -625,9 +624,9 @@ fn project_vector(vector: &Vector, opts: &ProjectionOptions) -> ProjectionResult
                 validity = merge_infinity_i32(src, Q_INF_DATE, Q_NINF_DATE, validity);
             }
             let nulls = to_null_buffer(validity);
-            let mut bytes_buf = vec![0u8; src.len() * size_of::<i32>()];
-            copy_and_offset_dates(src, bytemuck::cast_slice_mut(&mut bytes_buf));
-            let buf = Buffer::from_vec(bytes_buf);
+            let mut typed_buf = vec![0i32; src.len()];
+            copy_and_offset_dates(src, &mut typed_buf);
+            let buf = Buffer::from_vec(typed_buf);
             let arr: ArrayRef =
                 Arc::new(Date32Array::new(ScalarBuffer::new(buf, 0, src.len()), nulls));
             (arr, DataType::Date32)
@@ -667,9 +666,9 @@ fn project_vector(vector: &Vector, opts: &ProjectionOptions) -> ProjectionResult
                 validity = merge_infinity_i32(src, Q_INF_MINUTE, Q_NINF_MINUTE, validity);
             }
             let nulls = to_null_buffer(validity);
-            let mut bytes_buf = vec![0u8; src.len() * size_of::<i32>()];
-            copy_and_minutes_to_seconds(src, bytemuck::cast_slice_mut(&mut bytes_buf));
-            let buf = Buffer::from_vec(bytes_buf);
+            let mut typed_buf = vec![0i32; src.len()];
+            copy_and_minutes_to_seconds(src, &mut typed_buf);
+            let buf = Buffer::from_vec(typed_buf);
             let arr: ArrayRef = Arc::new(Time32SecondArray::new(
                 ScalarBuffer::new(buf, 0, src.len()),
                 nulls,

crates/qroissant-python/Cargo.toml

@@ -14,7 +14,7 @@ path = "src/lib.rs"
 bb8 = "0.9.0"
 bytes = "1.11.1"
 chrono = "0.4.44"
-pyo3 = { workspace = true, features = ["extension-module"] }
+pyo3 = { workspace = true, features = ["extension-module", "abi3-py311"] }
 pyo3-arrow = { version = "0.17.0", default-features = false }
 pyo3-async-runtimes = { version = "0.28.0", features = ["tokio-runtime"] }
 qroissant-arrow = { path = "../qroissant-arrow" }

crates/qroissant-python/src/values.rs

@@ -15,6 +15,7 @@ use pyo3::types::PyTuple;
 use pyo3_arrow::ffi::ArrayIterator;
 use pyo3_arrow::ffi::to_array_pycapsules;
 use pyo3_arrow::ffi::to_stream_pycapsule;
+use qroissant_arrow::HeterogeneousListMode;
 use qroissant_arrow::IngestionError;
 use qroissant_arrow::ProjectionOptions;
 use qroissant_arrow::ingest_array;
@@ -210,7 +211,7 @@ impl Atom {
         let schema_capsule: Option<Bound<'_, PyCapsule>> = requested_schema
             .map(|s| s.downcast_into::<PyCapsule>())
             .transpose()?;
-        let opts = slf.as_super().projection_opts().clone();
+        let opts = effective_export_opts(py, slf.as_super().projection_opts());
         let export = project(slf.as_super().inner(), &opts)
             .map_err(|e| PyNotImplementedError::new_err(e.to_string()))?;
         let capsules =
@@ -286,7 +287,7 @@ impl Vector {
         let schema_capsule: Option<Bound<'_, PyCapsule>> = requested_schema
             .map(|s| s.downcast_into::<PyCapsule>())
             .transpose()?;
-        let opts = slf.as_super().projection_opts().clone();
+        let opts = effective_export_opts(py, slf.as_super().projection_opts());
         let export = project(slf.as_super().inner(), &opts)
             .map_err(|e| PyNotImplementedError::new_err(e.to_string()))?;
         let capsules =
@@ -366,7 +367,7 @@ impl List {
         let schema_capsule: Option<Bound<'_, PyCapsule>> = requested_schema
             .map(|s| s.downcast_into::<PyCapsule>())
             .transpose()?;
-        let opts = slf.as_super().projection_opts().clone();
+        let opts = effective_export_opts(py, slf.as_super().projection_opts());
         let export = project(slf.as_super().inner(), &opts)
             .map_err(|e| PyNotImplementedError::new_err(e.to_string()))?;
         let capsules =
@@ -438,7 +439,7 @@ impl Dictionary {
         let schema_capsule: Option<Bound<'_, PyCapsule>> = requested_schema
             .map(|s| s.downcast_into::<PyCapsule>())
             .transpose()?;
-        let opts = slf.as_super().projection_opts().clone();
+        let opts = effective_export_opts(py, slf.as_super().projection_opts());
         let export = project(slf.as_super().inner(), &opts)
             .map_err(|e| PyNotImplementedError::new_err(e.to_string()))?;
         let capsules =
@@ -535,7 +536,7 @@ impl Table {
             qroissant_core::Value::Table(t) => t.clone(),
             _ => unreachable!("Table instances always hold q tables"),
         };
-        let opts = slf.as_super().projection_opts().clone();
+        let opts = effective_export_opts(py, slf.as_super().projection_opts());
         let export = py
             .detach(|| project_table(&table, &opts).map_err(|e| e.to_string()))
             .map_err(|e| PyNotImplementedError::new_err(e))?;
@@ -829,6 +830,34 @@ fn map_ingestion_error(e: IngestionError) -> PyErr {
     PyValueError::new_err(e.to_string())
 }
 
+/// If Polars is loaded in this Python process and the caller has not opted out
+/// of the default heterogeneous-list mode (still `Union`), transparently switch
+/// to `CoalesceTemporals` so mixed-precision temporal columns surface as a flat
+/// `Timestamp(ns)` instead of an Arrow `Union` (which Polars rejects with a
+/// hard panic).
+///
+/// Heuristic: presence of `polars` in `sys.modules` strongly correlates with a
+/// Polars consumer for the Arrow PyCapsule export. Users who want faithful
+/// `Union` despite Polars being imported should set the mode explicitly via
+/// `DecodeOptions.with_coalesce_temporals(False)` — left as a documented
+/// limitation, since explicit `Union` and the default are indistinguishable.
+fn effective_export_opts(py: Python<'_>, opts: &Arc<ProjectionOptions>) -> Arc<ProjectionOptions> {
+    if opts.heterogeneous_list_mode != HeterogeneousListMode::Union {
+        return opts.clone();
+    }
+    let polars_loaded = py
+        .import("sys")
+        .and_then(|sys| sys.getattr("modules"))
+        .and_then(|m| m.contains("polars"))
+        .unwrap_or(false);
+    if !polars_loaded {
+        return opts.clone();
+    }
+    let mut next = (**opts).clone();
+    next.heterogeneous_list_mode = HeterogeneousListMode::CoalesceTemporals;
+    Arc::new(next)
+}
+
 pub fn python_to_core_value(value: &Bound<'_, PyAny>) -> PyResult<CoreValue> {
     // Try qroissant Value first (it also implements Arrow protocols, so must come first).
     if let Ok(q_value) = value.extract::<PyRef<'_, Value>>() {

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "maturin"
 
 [project]
 name = "qroissant"
-version = "0.3.0"
+version = "0.3.1"
 description = "q/kdb+ IPC client library with Arrow-native Python interoperability"
 readme = "README.md"
 requires-python = ">=3.10"