BUG: unpickling modifies Block.ndim

doc/source/whatsnew/v1.2.0.rst

@@ -380,7 +380,7 @@ Categorical
 ^^^^^^^^^^^
 - :meth:`Categorical.fillna` will always return a copy, will validate a passed fill value regardless of whether there are any NAs to fill, and will disallow a ``NaT`` as a fill value for numeric categories (:issue:`36530`)
 - Bug in :meth:`Categorical.__setitem__` that incorrectly raised when trying to set a tuple value (:issue:`20439`)
--
+- Bug in indexing on a :class:`Series` with ``CategoricalDtype`` after unpickling (:issue:`37631`)
 
 Datetimelike
 ^^^^^^^^^^^^

pandas/core/internals/managers.py

@@ -278,14 +278,17 @@ def __getstate__(self):
         return axes_array, block_values, block_items, extra_state
 
     def __setstate__(self, state):
-        def unpickle_block(values, mgr_locs):
-            return make_block(values, placement=mgr_locs)
+        def unpickle_block(values, mgr_locs, ndim: int):
+            # TODO(EA2D): ndim would be unnecessary with 2D EAs
+            return make_block(values, placement=mgr_locs, ndim=ndim)
 
         if isinstance(state, tuple) and len(state) >= 4 and "0.14.1" in state[3]:
             state = state[3]["0.14.1"]
             self.axes = [ensure_index(ax) for ax in state["axes"]]
+            ndim = len(self.axes)
             self.blocks = tuple(
-                unpickle_block(b["values"], b["mgr_locs"]) for b in state["blocks"]
+                unpickle_block(b["values"], b["mgr_locs"], ndim=ndim)
+                for b in state["blocks"]
             )
         else:
             raise NotImplementedError("pre-0.14.1 pickles are no longer supported")

pandas/tests/io/test_pickle.py

@@ -576,3 +576,15 @@ def test_pickle_datetimes(datetime_series):
 def test_pickle_strings(string_series):
     unp_series = tm.round_trip_pickle(string_series)
     tm.assert_series_equal(unp_series, string_series)
+
+
+def test_pickle_preserves_block_ndim():
+    # GH#37631
+    ser = Series(list("abc")).astype("category").iloc[[0]]
+    res = tm.round_trip_pickle(ser)
+
+    assert res._mgr.blocks[0].ndim == 1
+    assert res._mgr.blocks[0].shape == (1,)
+
+    # GH#37631 OP issue was about indexing, underlying problem was pickle
+    tm.assert_series_equal(res[[True]], ser)