From 4b6ca3a95ad45e8be2f89ed7ed24bf8279ab48af Mon Sep 17 00:00:00 2001
From: RanaPriyansh <priyanshrana18@gmail.com>
Date: Mon, 28 Sep 2026 22:51:13 +0530
Subject: [PATCH] fix: keep chunk-cached DataArray.data lazy

---
 src/parcels/_chunk_cached_array/core.py |  2 +-
 tests/test_chunk_cached_array.py        | 50 +++++++++++++++++++++++++
 tests/test_interpolation.py             | 12 ++++++
 3 files changed, 63 insertions(+), 1 deletion(-)
 create mode 100644 tests/test_chunk_cached_array.py

diff --git a/src/parcels/_chunk_cached_array/core.py b/src/parcels/_chunk_cached_array/core.py
index bded6d1a5..520f48452 100644
--- a/src/parcels/_chunk_cached_array/core.py
+++ b/src/parcels/_chunk_cached_array/core.py
@@ -74,7 +74,7 @@ def __init__(self, dask_array: dask.array.Array, max_cache_bytes: int) -> None:
             self._boundaries.append(np.concatenate(([0], np.cumsum(dim_chunks))))
 
     def get_duck_array(self):
-        return self.array.compute()
+        return self.array
 
     def _raw_vindex(self, *indices: np.ndarray) -> np.ndarray:
         """Vectorized indexing with chunk caching.
diff --git a/tests/test_chunk_cached_array.py b/tests/test_chunk_cached_array.py
new file mode 100644
index 000000000..935c7e3e0
--- /dev/null
+++ b/tests/test_chunk_cached_array.py
@@ -0,0 +1,50 @@
+import dask
+import dask.array as da
+import numpy as np
+import xarray as xr
+
+from parcels._chunk_cached_array import ChunkCachedArray, wrap_dataset
+
+
+def test_chunk_cached_data_stays_lazy_until_explicit_materialization():
+    loaded = []
+
+    @dask.delayed
+    def chunk(index):
+        loaded.append(index)
+        return np.arange(16 * index, 16 * index + 16).reshape(4, 4)
+
+    array = da.concatenate([da.from_delayed(chunk(i), shape=(4, 4), dtype=int) for i in range(2)])
+    dataset = wrap_dataset(xr.Dataset({"value": (("x", "y"), array)}), max_cache_bytes=1024)
+
+    with dask.config.set(scheduler="synchronous"):
+        data = dataset.value.data
+        assert isinstance(data, da.Array)
+        assert loaded == []
+
+        np.testing.assert_array_equal(dataset.value.values, np.arange(32).reshape(8, 4))
+        assert sorted(loaded) == [0, 1]
+
+        loaded.clear()
+        np.testing.assert_array_equal(np.asarray(dataset.value), np.arange(32).reshape(8, 4))
+        assert sorted(loaded) == [0, 1]
+
+
+def test_chunk_cached_vectorized_selection_reuses_cached_chunk():
+    loaded = []
+
+    @dask.delayed
+    def chunk(index):
+        loaded.append(index)
+        return np.arange(16 * index, 16 * index + 16).reshape(4, 4)
+
+    array = da.concatenate([da.from_delayed(chunk(i), shape=(4, 4), dtype=int) for i in range(2)])
+    dataset = wrap_dataset(xr.Dataset({"value": (("x", "y"), array)}), max_cache_bytes=1024)
+    assert isinstance(dataset.value.variable._data, ChunkCachedArray)
+    indices = {"x": xr.DataArray([0, 1], dims="points"), "y": xr.DataArray([1, 2], dims="points")}
+
+    with dask.config.set(scheduler="synchronous"):
+        np.testing.assert_array_equal(dataset.value.isel(indices).data, [1, 6])
+        assert loaded == [0]
+        np.testing.assert_array_equal(dataset.value.isel(indices).data, [1, 6])
+        assert loaded == [0]
diff --git a/tests/test_interpolation.py b/tests/test_interpolation.py
index cfb690916..5773ee65f 100644
--- a/tests/test_interpolation.py
+++ b/tests/test_interpolation.py
@@ -13,6 +13,7 @@
     VectorField,
     particlefile_to_v3_zarr,
 )
+from parcels._chunk_cached_array import ChunkCachedArray
 from parcels._core.index_search import _search_time_index
 from parcels._core.mesh import get_mesh
 from parcels._datasets.structured.generated import simple_UV_dataset
@@ -118,6 +119,17 @@ def test_raw_2d_interpolation(field, interpolator, t, z, y, x, expected):
     np.testing.assert_equal(value, expected)
 
 
+def test_linear_interpolation_with_chunk_cached_data(field):
+    field.model.data = field.model.data.chunk({"time": 1, "depth": 1, "lat": 2, "lon": 2})
+    field.model.to_chunk_cached_arrays(max_cache_bytes=1024)
+    assert isinstance(field.data.variable._data, ChunkCachedArray)
+    particle_positions = {"time": [0, 1], "z": [0, 0], "lat": [0.49, 0.49], "lon": [0.51, 0.51]}
+    grid_positions = field.grid.search(particle_positions["z"], particle_positions["lat"], particle_positions["lon"])
+    grid_positions.update(_search_time_index(field, particle_positions["time"]))
+
+    np.testing.assert_allclose(field.interp_method.interp(particle_positions, grid_positions, field), [1.49, 6.49])
+
+
 @pytest.mark.parametrize("mesh", ["flat", "spherical"])
 @pytest.mark.parametrize(
     "func, t, z, y, x, expected",