BUG: Preserve CategoricalDtype._ordered_from_sentinel with pickle

pandas/core/dtypes/dtypes.py

@@ -219,7 +219,7 @@ class CategoricalDtype(PandasExtensionDtype, ExtensionDtype):
     kind = "O"  # type: str_type
     str = "|O08"
     base = np.dtype("O")
-    _metadata = ("categories", "ordered")
+    _metadata = ("categories", "ordered", "_ordered_from_sentinel")
     _cache = {}  # type: Dict[str_type, PandasExtensionDtype]
 
     def __init__(self, categories=None, ordered: OrderedType = ordered_sentinel):
@@ -356,6 +356,7 @@ def __setstate__(self, state: Dict[str_type, Any]) -> None:
         # pickle -> need to set the settable private ones here (see GH26067)
         self._categories = state.pop("categories", None)
         self._ordered = state.pop("ordered", False)
+        self._ordered_from_sentinel = state.pop("_ordered_from_sentinel", False)
 
     def __hash__(self) -> int:
         # _hash_categories returns a uint64, so use the negative

pandas/tests/dtypes/test_dtypes.py

@@ -903,6 +903,19 @@ def test_ordered_none_default_deprecated(self, ordered):
         with tm.assert_produces_warning(warning):
             dtype.ordered
 
+    @pytest.mark.parametrize("ordered", [True, False, None, ordered_sentinel])
+    def test_pickle_ordered_from_sentinel(self, ordered):
+        # GH 27295: can remove test when _ordered_from_sentinel is removed (GH 26403)
+        dtype = CategoricalDtype(categories=list("abc"), ordered=ordered)
+
+        warning = FutureWarning if ordered is ordered_sentinel else None
+        with tm.assert_produces_warning(warning, check_stacklevel=False):
+            dtype_from_pickle = tm.round_trip_pickle(dtype)
+
+        result = dtype_from_pickle._ordered_from_sentinel
+        expected = ordered is ordered_sentinel
+        assert result is expected
+
 
 @pytest.mark.parametrize(
     "dtype", [CategoricalDtype, IntervalDtype, DatetimeTZDtype, PeriodDtype]

pandas/tests/series/test_io.py

@@ -226,6 +226,15 @@ def test_pickle_preserve_name(self):
             unpickled = self._pickle_roundtrip_name(tm.makeTimeSeries(name=n))
             assert unpickled.name == n
 
+    def test_pickle_categorical_ordered_from_sentinel(self):
+        # GH 27295: can remove test when _ordered_from_sentinel is removed (GH 26403)
+        s = Series(["a", "b", "c", "a"], dtype="category")
+        result = tm.round_trip_pickle(s)
+        result = result.astype("category")
+
+        tm.assert_series_equal(result, s)
+        assert result.dtype._ordered_from_sentinel is False
+
     def _pickle_roundtrip_name(self, obj):
 
         with ensure_clean() as path: