From 01b0fe6f9b14ad1ad8216dff16a8cc4d1716c478 Mon Sep 17 00:00:00 2001
From: ajpotts <ajpotts@users.noreply.github.com>
Date: Mon, 1 Dec 2025 14:15:45 -0500
Subject: [PATCH] Part 2 of #5096:  unit tests for inherited functions in
 arkouda extension array

---
 pytest.ini                                    |   3 +-
 .../arkouda_array_extension_inherited.py      | 110 ++++++++++++++++++
 2 files changed, 111 insertions(+), 2 deletions(-)
 create mode 100644 tests/pandas/extension/arkouda_array_extension_inherited.py

diff --git a/pytest.ini b/pytest.ini
index 29c303aee43..9d05638d930 100644
--- a/pytest.ini
+++ b/pytest.ini
@@ -71,6 +71,7 @@ testpaths =
     tests/pandas/dataframe_test.py
     tests/pandas/extension/arithmetic_ops.py
     tests/pandas/extension/arkouda_array_extension.py
+    tests/pandas/extension/arkouda_array_extension_inherited.py
     tests/pandas/extension/arkouda_categorical_extension.py
     tests/pandas/extension/arkouda_extension.py
     tests/pandas/extension/arkouda_strings_extension.py
@@ -104,8 +105,6 @@ testpaths =
     tests/testing/equivalence_asserters_test.py
     tests/version_test.py
     tests/where_test.py
-
-
 norecursedirs =
     .git
     dist
diff --git a/tests/pandas/extension/arkouda_array_extension_inherited.py b/tests/pandas/extension/arkouda_array_extension_inherited.py
new file mode 100644
index 00000000000..80029f0a035
--- /dev/null
+++ b/tests/pandas/extension/arkouda_array_extension_inherited.py
@@ -0,0 +1,110 @@
+import numpy as np
+import pandas as pd
+import pytest
+
+from pandas.api.extensions import ExtensionArray as PandasExtensionArray
+
+import arkouda as ak
+
+from arkouda import Categorical
+from arkouda.pandas.extension import (
+    ArkoudaArray,
+    ArkoudaCategoricalArray,
+    ArkoudaStringArray,
+)
+
+
+class TestArkoudaArrayExplodeInherited:
+    def test_explode_is_inherited_from_pandas_extension_array(self):
+        """
+        ArkoudaArray should *not* override `_explode`; it should inherit the
+        default implementation from pandas.api.extensions.ExtensionArray.
+
+        This locks in the fact that explode semantics are coming from pandas,
+        not Arkouda-specific code.
+        """
+        # Not defined directly on ArkoudaArray
+        assert "_explode" not in ArkoudaArray.__dict__
+
+        # Method object is exactly the base ExtensionArray implementation
+        assert ArkoudaArray._explode is PandasExtensionArray._explode
+
+    @pytest.mark.parametrize(
+        "ea_cls, values",
+        [
+            # Numeric ArkoudaArray examples
+            (ArkoudaArray, [1, 2, 3]),
+            (ArkoudaArray, [10.5, 20.5]),
+            # String-backed example
+            (ArkoudaStringArray, ["a", "b", "c"]),
+            # Categorical-backed example
+            (ArkoudaCategoricalArray, ["red", "blue", "red"]),
+        ],
+    )
+    def test_explode_scalar_data_roundtrip(self, ea_cls, values):
+        """
+        For arrays that do NOT contain list-like elements, the default
+        ExtensionArray._explode implementation should:
+
+        - return a copy of the array (same values, same type),
+        - return lengths as an ndarray of ones with shape (len(arr),).
+
+        This should hold for ArkoudaArray, ArkoudaStringsArray, and
+        ArkoudaCategoricalArray when they wrap scalar (non-list-like) data.
+        """
+        # Construct the underlying Arkouda data depending on EA type
+        if ea_cls is ArkoudaCategoricalArray:
+            # Categorical typically wraps an Arkouda Categorical built from values
+            ak_data = Categorical(ak.array(values))
+        else:
+            # Numeric or string cases: ak.array should produce the right Arkouda type
+            ak_data = ak.array(values)
+
+        arr = ea_cls(ak_data)
+
+        exploded, lengths = arr._explode()
+
+        # 1) Same EA type
+        assert isinstance(exploded, type(arr))
+
+        # 2) Values unchanged
+        orig_np = arr.to_numpy()
+        exploded_np = exploded.to_numpy()
+        np.testing.assert_array_equal(exploded_np, orig_np)
+
+        # 3) lengths: ndarray of ones, one entry per original element
+        assert isinstance(lengths, np.ndarray)
+        assert lengths.shape == (len(arr),)
+        assert np.issubdtype(lengths.dtype, np.integer)
+        assert np.all(lengths == 1)
+
+    @pytest.mark.parametrize(
+        "ea_cls, values",
+        [
+            (ArkoudaArray, [1, 2, 3]),
+            (ArkoudaStringArray, ["x", "y", "z"]),
+            (ArkoudaCategoricalArray, ["low", "med", "high"]),
+        ],
+    )
+    def test_explode_does_not_change_series_values(self, ea_cls, values):
+        """
+        Series.explode for non-list-like, non-object extension dtypes is
+        effectively a no-op: it returns a Series copy with identical data
+        and index. Arkouda-backed Series of all three EA types should
+        follow that behavior.
+        """
+        if ea_cls is ArkoudaCategoricalArray:
+            ak_data = Categorical(ak.array(values))
+        else:
+            ak_data = ak.array(values)
+
+        arr = ea_cls(ak_data)
+        s = pd.Series(arr)
+
+        result = s.explode()
+
+        # Expect a value-wise identical Series.
+        # (dtype may differ depending on pandas internals, so we focus on
+        # values and index equality.)
+        assert list(result.index) == list(s.index)
+        np.testing.assert_array_equal(result.to_numpy(), s.to_numpy())