Backport PR #55363 on branch 2.1.x (BUG: ndim of string block incorre…

…ct with string inference) (#55376)

Backport PR #55363: BUG: ndim of string block incorrect with string inference

Co-authored-by: Patrick Hoefler <61934744+phofl@users.noreply.github.com>

pandas/core/internals/construction.py

@@ -383,7 +383,7 @@ def ndarray_to_mgr(
             new_block(
                 dtype.construct_array_type()._from_sequence(data, dtype=dtype),
                 BlockPlacement(slice(i, i + 1)),
-                ndim=1,
+                ndim=2,
             )
             for i, data in enumerate(obj_columns)
         ]

pandas/tests/frame/test_constructors.py

@@ -2743,6 +2743,13 @@ def test_frame_string_inference_array_string_dtype(self):
             df = DataFrame(np.array([["a", "c"], ["b", "d"]]), columns=["a", "b"])
         tm.assert_frame_equal(df, expected)
 
+    def test_frame_string_inference_block_dim(self):
+        # GH#55363
+        pytest.importorskip("pyarrow")
+        with pd.option_context("future.infer_string", True):
+            df = DataFrame(np.array([["hello", "goodbye"], ["hello", "Hello"]]))
+        assert df._mgr.blocks[0].ndim == 2
+
 
 class TestDataFrameConstructorIndexInference:
     def test_frame_from_dict_of_series_overlapping_monthly_period_indexes(self):