BUG: Impossible creation of array with dtype=string (#61263)

* DOC: Update warning in Index.values docstring to clarify index modification issues (#60954)

* DOC: Update warning in Index.values docstring to clarify index modification issues (#60954) with changes

* Update pandas/core/indexes/base.py

Co-authored-by: Matthew Roeschke <[email protected]>

* DOC : Fixing the whitespace which was causing error

* Fixed docstring validation and formatting issues

* BUG: Fix array creation for string dtype with inconsistent list lengths (#61155)

* BUG: Fix array creation for string dtype with inconsistent list lengths (#61155)

* BUG fix GH#61155 v2

* BUG fix GH#61155 with test case for list of lists handling

* Fix formatting in test_string_array.py (pre-commit autofix)

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Add test for list of lists handling in ensure_string_array (GH#61155)

* fixing checks

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Update pandas/tests/libs/test_lib.py

Co-authored-by: Richard Shadrach <[email protected]>

* Remove pandas/tests/arrays/test_string_array.py as requested

* wrong fiel base.py

* Remove check for nested lists in scalars in string_.py first try

* Revert unintended changes to base.py

---------

Co-authored-by: Matthew Roeschke <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Richard Shadrach <[email protected]>

Author: 4 people

pandas/_libs/lib.pyx

@@ -777,7 +777,10 @@ cpdef ndarray[object] ensure_string_array(
             return out
         arr = arr.to_numpy(dtype=object)
     elif not util.is_array(arr):
-        arr = np.array(arr, dtype="object")
+        # GH#61155: Guarantee a 1-d result when array is a list of lists
+        input_arr = arr
+        arr = np.empty(len(arr), dtype="object")
+        arr[:] = input_arr
 
     result = np.asarray(arr, dtype="object")
 

pandas/tests/libs/test_lib.py

@@ -297,3 +297,13 @@ def test_ensure_string_array_copy():
     assert not np.shares_memory(arr, result)
     assert arr[1] is None
     assert result[1] is np.nan
+
+
+def test_ensure_string_array_list_of_lists():
+    # GH#61155: ensure list of lists doesn't get converted to string
+    arr = [list("test"), list("word")]
+    result = lib.ensure_string_array(arr)
+
+    # Each item in result should still be a list, not a stringified version
+    expected = np.array(["['t', 'e', 's', 't']", "['w', 'o', 'r', 'd']"], dtype=object)
+    tm.assert_numpy_array_equal(result, expected)