BUG: Series.str.split(expand=True) for ArrowDtype(pa.string()) (#53532)

* BUG: Series.str.split(expand=True) for ArrowDtype(pa.string())

* whatsnew

* min versions

* ensure ArrowExtensionArray

Author: lukemanley

doc/source/whatsnew/v2.0.3.rst

@@ -22,6 +22,8 @@ Fixed regressions
 Bug fixes
 ~~~~~~~~~
 - Bug in :func:`read_csv` when defining ``dtype`` with ``bool[pyarrow]`` for the ``"c"`` and ``"python"`` engines (:issue:`53390`)
+- Bug in :meth:`Series.str.split` and :meth:`Series.str.rsplit` with ``expand=True`` for :class:`ArrowDtype` with ``pyarrow.string`` (:issue:`53532`)
+-
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_203.other:

pandas/core/strings/accessor.py

@@ -275,14 +275,40 @@ def _wrap_result(
             if isinstance(result.dtype, ArrowDtype):
                 import pyarrow as pa
 
+                from pandas.compat import pa_version_under11p0
+
                 from pandas.core.arrays.arrow.array import ArrowExtensionArray
 
-                max_len = pa.compute.max(
-                    result._pa_array.combine_chunks().value_lengths()
-                ).as_py()
-                if result.isna().any():
+                value_lengths = result._pa_array.combine_chunks().value_lengths()
+                max_len = pa.compute.max(value_lengths).as_py()
+                min_len = pa.compute.min(value_lengths).as_py()
+                if result._hasna:
                     # ArrowExtensionArray.fillna doesn't work for list scalars
-                    result._pa_array = result._pa_array.fill_null([None] * max_len)
+                    result = ArrowExtensionArray(
+                        result._pa_array.fill_null([None] * max_len)
+                    )
+                if min_len < max_len:
+                    # append nulls to each scalar list element up to max_len
+                    if not pa_version_under11p0:
+                        result = ArrowExtensionArray(
+                            pa.compute.list_slice(
+                                result._pa_array,
+                                start=0,
+                                stop=max_len,
+                                return_fixed_size_list=True,
+                            )
+                        )
+                    else:
+                        all_null = np.full(max_len, fill_value=None, dtype=object)
+                        values = result.to_numpy()
+                        new_values = []
+                        for row in values:
+                            if len(row) < max_len:
+                                nulls = all_null[: max_len - len(row)]
+                                row = np.append(row, nulls)
+                            new_values.append(row)
+                        pa_type = result._pa_array.type
+                        result = ArrowExtensionArray(pa.array(new_values, type=pa_type))
                 if name is not None:
                     labels = name
                 else:

pandas/tests/extension/test_arrow.py

@@ -2286,6 +2286,15 @@ def test_str_split():
     )
     tm.assert_frame_equal(result, expected)
 
+    result = ser.str.split("1", expand=True)
+    expected = pd.DataFrame(
+        {
+            0: ArrowExtensionArray(pa.array(["a", "a2cbcb", None])),
+            1: ArrowExtensionArray(pa.array(["cbcb", None, None])),
+        }
+    )
+    tm.assert_frame_equal(result, expected)
+
 
 def test_str_rsplit():
     # GH 52401
@@ -2311,6 +2320,15 @@ def test_str_rsplit():
     )
     tm.assert_frame_equal(result, expected)
 
+    result = ser.str.rsplit("1", expand=True)
+    expected = pd.DataFrame(
+        {
+            0: ArrowExtensionArray(pa.array(["a", "a2cbcb", None])),
+            1: ArrowExtensionArray(pa.array(["cbcb", None, None])),
+        }
+    )
+    tm.assert_frame_equal(result, expected)
+
 
 def test_str_unsupported_extract():
     ser = pd.Series(["abc", None], dtype=ArrowDtype(pa.string()))