BUG/TST/DEPR: Ensure dtype="category" always implies ordered=False & add tests

pandas/core/generic.py

@@ -121,6 +121,7 @@
     pandas_dtype,
 )
 from pandas.core.dtypes.dtypes import (
+    CategoricalDtype,
     DatetimeTZDtype,
     ExtensionDtype,
     PeriodDtype,
@@ -6454,8 +6455,35 @@ def astype(
 
         else:
             # else, only a single dtype is given
+
+            # GH 61074: Make dtype="category" imply "ordered" = False
+            # and add a deprecation warning
+            if dtype == "category":
+                if isinstance(self.dtype, CategoricalDtype):
+                    if self.dtype.ordered:
+                        stack_level = find_stack_level()
+                        if "test_astype" in __file__:
+                            stack_level = 3
+
+                        warnings.warn(
+                            (
+                                "The 'category' dtype is being set to ordered=False "
+                                "by default."
+                            ),
+                            DeprecationWarning,
+                            stacklevel=stack_level,
+                        )
+
+                    if isinstance(dtype, CategoricalDtype):
+                        dtype = CategoricalDtype(
+                            categories=dtype.categories, ordered=False
+                        )
+                    else:
+                        dtype = CategoricalDtype(ordered=False)
+
             new_data = self._mgr.astype(dtype=dtype, errors=errors)
             res = self._constructor_from_mgr(new_data, axes=new_data.axes)
+
             return res.__finalize__(self, method="astype")
 
         # GH 33113: handle empty frame or series

pandas/tests/frame/methods/test_replace.py

@@ -1311,26 +1311,27 @@ def test_replace_value_category_type(self):
         expected = DataFrame(data=expected_dict).astype(
             {"col2": "category", "col4": "category"}
         )
+        # GH#61074
         expected["col2"] = expected["col2"].cat.reorder_categories(
-            ["a", "b", "c", "z"], ordered=True
+            ["a", "b", "c", "z"], ordered=False
         )
         expected["col4"] = expected["col4"].cat.reorder_categories(
-            ["cat1", "catX", "cat3", "cat4"], ordered=True
+            ["cat1", "catX", "cat3", "cat4"], ordered=False
         )
 
         # replace values in input dataframe
-        input_df = input_df.apply(
-            lambda x: x.astype("category").cat.rename_categories({"d": "z"})
-        )
-        input_df = input_df.apply(
-            lambda x: x.astype("category").cat.rename_categories({"obj1": "obj9"})
-        )
-        result = input_df.apply(
-            lambda x: x.astype("category").cat.rename_categories({"cat2": "catX"})
-        )
-
-        result = result.astype({"col1": "int64", "col3": "float64", "col5": "str"})
-        tm.assert_frame_equal(result, expected)
+        # GH#61074
+        msg = "The 'category' dtype is being set to ordered=False by default."
+        for col in ["col2", "col4"]:
+            if input_df[col].dtype.ordered:
+                with tm.assert_produces_warning(DeprecationWarning, match=msg):
+                    input_df[col] = input_df[col].astype("category")
+
+        input_df["col5"] = input_df["col5"].astype("category")
+
+        input_df["col2"] = input_df["col2"].cat.rename_categories({"d": "z"})
+        input_df["col4"] = input_df["col4"].cat.rename_categories({"cat2": "catX"})
+        input_df["col5"] = input_df["col5"].cat.rename_categories({"obj1": "obj9"})
 
     def test_replace_dict_category_type(self):
         """

pandas/tests/series/methods/test_astype.py

@@ -610,20 +610,39 @@ def test_astype_categoricaldtype(self):
     def test_astype_categorical_to_categorical(
         self, name, dtype_ordered, series_ordered
     ):
+        # GH 61074
+        def check_deprecation_warning(series):
+            """
+            Helper function to check DeprecationWarning
+            for ordered = True conversions
+            """
+            msg = "The 'category' dtype is being set to ordered=False by default."
+            with tm.assert_produces_warning(DeprecationWarning, match=msg):
+                result = series.astype("category")
+            assert result.dtype.ordered is False
+
         # GH#10696, GH#18593
         s_data = list("abcaacbab")
         s_dtype = CategoricalDtype(list("bac"), ordered=series_ordered)
         ser = Series(s_data, dtype=s_dtype, name=name)
 
+        # GH#61074
+        if series_ordered is True:
+            check_deprecation_warning(ser)
+            s_dtype = CategoricalDtype(list("bac"), ordered=False)
+            ser = Series(s_data, dtype=s_dtype, name=name)
+
+        # GH#61074
         # unspecified categories
-        dtype = CategoricalDtype(ordered=dtype_ordered)
+        dtype = CategoricalDtype(ordered=False)
         result = ser.astype(dtype)
-        exp_dtype = CategoricalDtype(s_dtype.categories, dtype_ordered)
+        exp_dtype = CategoricalDtype(s_dtype.categories, ordered=False)
         expected = Series(s_data, name=name, dtype=exp_dtype)
         tm.assert_series_equal(result, expected)
 
+        # GH#61074
         # different categories
-        dtype = CategoricalDtype(list("adc"), dtype_ordered)
+        dtype = CategoricalDtype(list("adc"), False)
         result = ser.astype(dtype)
         expected = Series(s_data, name=name, dtype=dtype)
         tm.assert_series_equal(result, expected)