DEPR: Deprecate ordered=None for CategoricalDtype (#26403)

doc/source/whatsnew/v0.23.0.rst

@@ -935,13 +935,23 @@ In previous versions, the default value for the ``ordered`` parameter was ``Fals
 
 New behavior:
 
-.. ipython:: python
+.. code-block:: ipython
 
-    from pandas.api.types import CategoricalDtype
-    cat = pd.Categorical(list('abcaba'), ordered=True, categories=list('cba'))
-    cat
-    cdt = CategoricalDtype(categories=list('cbad'))
-    cat.astype(cdt)
+    In [2]: from pandas.api.types import CategoricalDtype
+
+    In [3]: cat = pd.Categorical(list('abcaba'), ordered=True, categories=list('cba'))
+
+    In [4]: cat
+    Out[4]:
+    [a, b, c, a, b, a]
+    Categories (3, object): [c < b < a]
+
+    In [5]: cdt = CategoricalDtype(categories=list('cbad'))
+
+    In [6]: cat.astype(cdt)
+    Out[6]:
+    [a, b, c, a, b, a]
+    Categories (4, object): [c < b < a < d]
 
 Notice in the example above that the converted ``Categorical`` has retained ``ordered=True``.  Had the default value for ``ordered`` remained as ``False``, the converted ``Categorical`` would have become unordered, despite ``ordered=False`` never being explicitly specified.  To change the value of ``ordered``, explicitly pass it to the new dtype, e.g. ``CategoricalDtype(categories=list('cbad'), ordered=False)``.
 

doc/source/whatsnew/v0.25.0.rst

@@ -761,6 +761,7 @@ Other deprecations
 - :attr:`Series.imag` and :attr:`Series.real` are deprecated. (:issue:`18262`)
 - :meth:`Series.put` is deprecated. (:issue:`18262`)
 - :meth:`Index.item` and :meth:`Series.item` is deprecated. (:issue:`18262`)
+- The default value ``ordered=None`` in :class:`~pandas.api.types.CategoricalDtype` has been deprecated in favor of ``ordered=False``. When converting between categorical types ``ordered=True`` must be explicitly passed in order to be preserved. (:issue:`26336`)
 - :meth:`Index.contains` is deprecated. Use ``key in index`` (``__contains__``) instead (:issue:`17753`).
 
 .. _whatsnew_0250.prior_deprecations:

pandas/core/arrays/categorical.py

@@ -332,7 +332,7 @@ def __init__(self, values, categories=None, ordered=None, dtype=None,
         # sanitize input
         if is_categorical_dtype(values):
             if dtype.categories is None:
-                dtype = CategoricalDtype(values.categories, dtype.ordered)
+                dtype = CategoricalDtype(values.categories, dtype._ordered)
         elif not isinstance(values, (ABCIndexClass, ABCSeries)):
             # sanitize_array coerces np.nan to a string under certain versions
             # of numpy
@@ -355,7 +355,7 @@ def __init__(self, values, categories=None, ordered=None, dtype=None,
                 codes, categories = factorize(values, sort=True)
             except TypeError:
                 codes, categories = factorize(values, sort=False)
-                if dtype.ordered:
+                if dtype._ordered:
                     # raise, as we don't have a sortable data structure and so
                     # the user should give us one by specifying categories
                     raise TypeError("'values' is not ordered, please "
@@ -368,7 +368,7 @@ def __init__(self, values, categories=None, ordered=None, dtype=None,
                                           "supported at this time")
 
             # we're inferring from values
-            dtype = CategoricalDtype(categories, dtype.ordered)
+            dtype = CategoricalDtype(categories, dtype._ordered)
 
         elif is_categorical_dtype(values):
             old_codes = (values._values.codes if isinstance(values, ABCSeries)
@@ -433,7 +433,7 @@ def ordered(self):
         """
         Whether the categories have an ordered relationship.
         """
-        return self.dtype.ordered
+        return self.dtype._ordered
 
     @property
     def dtype(self) -> CategoricalDtype:
@@ -847,7 +847,7 @@ def set_categories(self, new_categories, ordered=None, rename=False,
         """
         inplace = validate_bool_kwarg(inplace, 'inplace')
         if ordered is None:
-            ordered = self.dtype.ordered
+            ordered = self.dtype._ordered
         new_dtype = CategoricalDtype(new_categories, ordered=ordered)
 
         cat = self if inplace else self.copy()

pandas/core/dtypes/dtypes.py

@@ -17,6 +17,13 @@
 
 str_type = str
 
+# GH26403: sentinel value used for the default value of ordered in the
+# CategoricalDtype constructor to detect when ordered=None is explicitly passed
+ordered_sentinel = object()  # type: object
+
+# TODO(GH26403): Replace with Optional[bool] or bool
+OrderedType = Union[None, bool, object]
+
 
 def register_extension_dtype(cls: Type[ExtensionDtype],
                              ) -> Type[ExtensionDtype]:
@@ -214,7 +221,9 @@ class CategoricalDtype(PandasExtensionDtype, ExtensionDtype):
     _metadata = ('categories', 'ordered')
     _cache = {}  # type: Dict[str_type, PandasExtensionDtype]
 
-    def __init__(self, categories=None, ordered: Optional[bool] = None):
+    def __init__(self,
+                 categories=None,
+                 ordered: OrderedType = ordered_sentinel):
         self._finalize(categories, ordered, fastpath=False)
 
     @classmethod
@@ -230,7 +239,7 @@ def _from_fastpath(cls,
     def _from_categorical_dtype(cls,
                                 dtype: 'CategoricalDtype',
                                 categories=None,
-                                ordered: Optional[bool] = None,
+                                ordered: OrderedType = None,
                                 ) -> 'CategoricalDtype':
         if categories is ordered is None:
             return dtype
@@ -330,19 +339,20 @@ def _from_values_or_dtype(cls,
 
     def _finalize(self,
                   categories,
-                  ordered: Optional[bool],
+                  ordered: OrderedType,
                   fastpath: bool = False,
                   ) -> None:
 
-        if ordered is not None:
+        if ordered is not None and ordered is not ordered_sentinel:
             self.validate_ordered(ordered)
 
         if categories is not None:
             categories = self.validate_categories(categories,
                                                   fastpath=fastpath)
 
         self._categories = categories
-        self._ordered = ordered
+        self._ordered = ordered if ordered is not ordered_sentinel else None
+        self._ordered_from_sentinel = ordered is ordered_sentinel
 
     def __setstate__(self, state: Dict[str_type, Any]) -> None:
         # for pickle compat. __get_state__ is defined in the
@@ -355,12 +365,12 @@ def __hash__(self) -> int:
         # _hash_categories returns a uint64, so use the negative
         # space for when we have unknown categories to avoid a conflict
         if self.categories is None:
-            if self.ordered:
+            if self._ordered:
                 return -1
             else:
                 return -2
         # We *do* want to include the real self.ordered here
-        return int(self._hash_categories(self.categories, self.ordered))
+        return int(self._hash_categories(self.categories, self._ordered))
 
     def __eq__(self, other: Any) -> bool:
         """
@@ -379,7 +389,7 @@ def __eq__(self, other: Any) -> bool:
             return other == self.name
         elif other is self:
             return True
-        elif not (hasattr(other, 'ordered') and hasattr(other, 'categories')):
+        elif not (hasattr(other, '_ordered') and hasattr(other, 'categories')):
             return False
         elif self.categories is None or other.categories is None:
             # We're forced into a suboptimal corner thanks to math and
@@ -388,10 +398,10 @@ def __eq__(self, other: Any) -> bool:
             # CDT(., .) = CDT(None, False) and *all*
             # CDT(., .) = CDT(None, True).
             return True
-        elif self.ordered or other.ordered:
+        elif self._ordered or other._ordered:
             # At least one has ordered=True; equal if both have ordered=True
             # and the same values for categories in the same order.
-            return ((self.ordered == other.ordered) and
+            return ((self._ordered == other._ordered) and
                     self.categories.equals(other.categories))
         else:
             # Neither has ordered=True; equal if both have the same categories,
@@ -406,10 +416,10 @@ def __repr__(self):
             data = "None, "
         else:
             data = self.categories._format_data(name=self.__class__.__name__)
-        return tpl.format(data, self.ordered)
+        return tpl.format(data, self._ordered)
 
     @staticmethod
-    def _hash_categories(categories, ordered: Optional[bool] = True) -> int:
+    def _hash_categories(categories, ordered: OrderedType = True) -> int:
         from pandas.core.util.hashing import (
             hash_array, _combine_hash_arrays, hash_tuples
         )
@@ -459,7 +469,7 @@ def construct_array_type(cls):
         return Categorical
 
     @staticmethod
-    def validate_ordered(ordered: bool) -> None:
+    def validate_ordered(ordered: OrderedType) -> None:
         """
         Validates that we have a valid ordered parameter. If
         it is not a boolean, a TypeError will be raised.
@@ -534,17 +544,25 @@ def update_dtype(self, dtype: 'CategoricalDtype') -> 'CategoricalDtype':
             msg = ('a CategoricalDtype must be passed to perform an update, '
                    'got {dtype!r}').format(dtype=dtype)
             raise ValueError(msg)
-        elif dtype.categories is not None and dtype.ordered is self.ordered:
-            return dtype
 
         # dtype is CDT: keep current categories/ordered if None
         new_categories = dtype.categories
         if new_categories is None:
             new_categories = self.categories
 
-        new_ordered = dtype.ordered
+        new_ordered = dtype._ordered
+        new_ordered_from_sentinel = dtype._ordered_from_sentinel
         if new_ordered is None:
-            new_ordered = self.ordered
+            # maintain existing ordered if new dtype has ordered=None
+            new_ordered = self._ordered
+            if self._ordered and new_ordered_from_sentinel:
+                # only warn if we'd actually change the existing behavior
+                msg = ("Constructing a CategoricalDtype without specifying "
+                       "`ordered` will default to `ordered=False` in a future "
+                       "version, which will cause the resulting categorical's "
+                       "`ordered` attribute to change to False; `ordered=True`"
+                       " must be explicitly passed in order to be retained")
+                warnings.warn(msg, FutureWarning, stacklevel=3)
 
         return CategoricalDtype(new_categories, new_ordered)
 
@@ -556,10 +574,18 @@ def categories(self):
         return self._categories
 
     @property
-    def ordered(self) -> Optional[bool]:
+    def ordered(self) -> OrderedType:
         """
         Whether the categories have an ordered relationship.
         """
+        # TODO: remove if block when ordered=None as default is deprecated
+        if self._ordered_from_sentinel and self._ordered is None:
+            # warn when accessing ordered if ordered=None and None was not
+            # explicitly passed to the constructor
+            msg = ("Constructing a CategoricalDtype without specifying "
+                   "`ordered` will default to `ordered=False` in a future "
+                   "version; `ordered=None` must be explicitly passed.")
+            warnings.warn(msg, FutureWarning, stacklevel=2)
         return self._ordered
 
     @property

pandas/core/internals/construction.py

@@ -725,7 +725,7 @@ def _try_cast(arr, dtype, copy, raise_cast_failure):
             # We *do* allow casting to categorical, since we know
             # that Categorical is the only array type for 'category'.
             subarr = Categorical(arr, dtype.categories,
-                                 ordered=dtype.ordered)
+                                 ordered=dtype._ordered)
         elif is_extension_array_dtype(dtype):
             # create an extension array from its dtype
             array_type = dtype.construct_array_type()._from_sequence

pandas/core/series.py

@@ -19,7 +19,7 @@
 from pandas.util._validators import validate_bool_kwarg
 
 from pandas.core.dtypes.common import (
-    _is_unorderable_exception, ensure_platform_int, is_bool,
+    _is_unorderable_exception, ensure_platform_int, is_bool, is_categorical,
     is_categorical_dtype, is_datetime64_dtype, is_datetimelike, is_dict_like,
     is_extension_array_dtype, is_extension_type, is_hashable, is_integer,
     is_iterator, is_list_like, is_scalar, is_string_like, is_timedelta64_dtype)
@@ -170,6 +170,12 @@ def __init__(self, data=None, index=None, dtype=None, name=None,
             if data is None:
                 data = {}
             if dtype is not None:
+                # GH 26336: explicitly handle 'category' to avoid warning
+                # TODO: Remove after CategoricalDtype defaults to ordered=False
+                if (isinstance(dtype, str) and dtype == 'category' and
+                        is_categorical(data)):
+                    dtype = data.dtype
+
                 dtype = self._validate_dtype(dtype)
 
             if isinstance(data, MultiIndex):

pandas/io/packers.py

@@ -618,14 +618,9 @@ def decode(obj):
         return Interval(obj['left'], obj['right'], obj['closed'])
     elif typ == 'series':
         dtype = dtype_for(obj['dtype'])
-        pd_dtype = pandas_dtype(dtype)
-
         index = obj['index']
-        result = Series(unconvert(obj['data'], dtype, obj['compress']),
-                        index=index,
-                        dtype=pd_dtype,
-                        name=obj['name'])
-        return result
+        data = unconvert(obj['data'], dtype, obj['compress'])
+        return Series(data, index=index, dtype=dtype, name=obj['name'])
 
     elif typ == 'block_manager':
         axes = obj['axes']

pandas/tests/arrays/categorical/test_dtypes.py

@@ -160,6 +160,14 @@ def test_astype_category(self, dtype_ordered, cat_ordered):
             expected = cat
             tm.assert_categorical_equal(result, expected)
 
+    def test_astype_category_ordered_none_deprecated(self):
+        # GH 26336
+        cdt1 = CategoricalDtype(categories=list('cdab'), ordered=True)
+        cdt2 = CategoricalDtype(categories=list('cedafb'))
+        cat = Categorical(list('abcdaba'), dtype=cdt1)
+        with tm.assert_produces_warning(FutureWarning):
+            cat.astype(cdt2)
+
     def test_iter_python_types(self):
         # GH-19909
         cat = Categorical([1, 2])

pandas/tests/dtypes/test_dtypes.py

@@ -10,7 +10,8 @@
     is_datetime64tz_dtype, is_datetimetz, is_dtype_equal, is_interval_dtype,
     is_period, is_period_dtype, is_string_dtype)
 from pandas.core.dtypes.dtypes import (
-    CategoricalDtype, DatetimeTZDtype, IntervalDtype, PeriodDtype, registry)
+    CategoricalDtype, DatetimeTZDtype, IntervalDtype, PeriodDtype,
+    ordered_sentinel, registry)
 
 import pandas as pd
 from pandas import (
@@ -54,7 +55,8 @@ def test_pickle(self):
 class TestCategoricalDtype(Base):
 
     def create(self):
-        return CategoricalDtype()
+        # TODO(GH 26403): Remove when default ordered becomes False
+        return CategoricalDtype(ordered=None)
 
     def test_pickle(self):
         # make sure our cache is NOT pickled
@@ -675,7 +677,8 @@ def test_unordered_same(self, ordered):
     def test_categories(self):
         result = CategoricalDtype(['a', 'b', 'c'])
         tm.assert_index_equal(result.categories, pd.Index(['a', 'b', 'c']))
-        assert result.ordered is None
+        with tm.assert_produces_warning(FutureWarning):
+            assert result.ordered is None
 
     def test_equal_but_different(self, ordered_fixture):
         c1 = CategoricalDtype([1, 2, 3])
@@ -804,7 +807,8 @@ def test_categorical_categories(self):
 
     @pytest.mark.parametrize('new_categories', [
         list('abc'), list('cba'), list('wxyz'), None])
-    @pytest.mark.parametrize('new_ordered', [True, False, None])
+    @pytest.mark.parametrize('new_ordered', [
+        True, False, None, ordered_sentinel])
     def test_update_dtype(self, ordered_fixture, new_categories, new_ordered):
         dtype = CategoricalDtype(list('abc'), ordered_fixture)
         new_dtype = CategoricalDtype(new_categories, new_ordered)
@@ -813,11 +817,18 @@ def test_update_dtype(self, ordered_fixture, new_categories, new_ordered):
         if expected_categories is None:
             expected_categories = dtype.categories
 
-        expected_ordered = new_dtype.ordered
-        if expected_ordered is None:
+        expected_ordered = new_ordered
+        if new_ordered is ordered_sentinel or new_ordered is None:
             expected_ordered = dtype.ordered
 
-        result = dtype.update_dtype(new_dtype)
+        # GH 26336
+        if new_ordered is ordered_sentinel and ordered_fixture is True:
+            with tm.assert_produces_warning(FutureWarning,
+                                            check_stacklevel=False):
+                result = dtype.update_dtype(new_dtype)
+        else:
+            result = dtype.update_dtype(new_dtype)
+
         tm.assert_index_equal(result.categories, expected_categories)
         assert result.ordered is expected_ordered
 
@@ -837,6 +848,14 @@ def test_update_dtype_errors(self, bad_dtype):
         with pytest.raises(ValueError, match=msg):
             dtype.update_dtype(bad_dtype)
 
+    @pytest.mark.parametrize('ordered', [ordered_sentinel, None, True, False])
+    def test_ordered_none_default_deprecated(self, ordered):
+        # GH 26403: CDT.ordered only warns if ordered is not explicitly passed
+        dtype = CategoricalDtype(list('abc'), ordered=ordered)
+        warning = FutureWarning if ordered is ordered_sentinel else None
+        with tm.assert_produces_warning(warning):
+            dtype.ordered
+
 
 @pytest.mark.parametrize('dtype', [
     CategoricalDtype,