BUG: Preserve CategoricalDtype._ordered_from_sentinel with pickle (#27317)

Author: jschendel

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: