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pandas/conftest.py

@@ -1057,3 +1057,17 @@ def index_or_series_obj(request):
     copy to avoid mutation, e.g. setting .name
     """
     return _index_or_series_objs[request.param].copy(deep=True)
+
+
+@pytest.fixture
+def multiindex_year_month_day_dataframe_random_data():
+    """
+    DataFrame with 3 level MultiIndex (year, month, day) covering
+    first 100 business days from 2000-01-01 with random data
+    """
+    tdf = tm.makeTimeDataFrame(100)
+    ymd = tdf.groupby([lambda x: x.year, lambda x: x.month, lambda x: x.day]).sum()
+    # use Int64Index, to make sure things work
+    ymd.index.set_levels([lev.astype("i8") for lev in ymd.index.levels], inplace=True)
+    ymd.index.set_names(["year", "month", "day"], inplace=True)
+    return ymd

pandas/core/dtypes/dtypes.py

@@ -175,6 +175,8 @@ class CategoricalDtype(PandasExtensionDtype, ExtensionDtype):
     ----------
     categories : sequence, optional
         Must be unique, and must not contain any nulls.
+        The categories are stored in an Index,
+        and if an index is provided the dtype of that index will be used.
     ordered : bool or None, default False
         Whether or not this categorical is treated as a ordered categorical.
         None can be used to maintain the ordered value of existing categoricals when
@@ -210,6 +212,12 @@ class CategoricalDtype(PandasExtensionDtype, ExtensionDtype):
     3    NaN
     dtype: category
     Categories (2, object): [b < a]
+
+    An empty CategoricalDtype with a specific dtype can be created
+    by providing an empty index. As follows,
+
+    >>> pd.CategoricalDtype(pd.DatetimeIndex([])).categories.dtype
+    dtype('<M8[ns]')
     """
 
     # TODO: Document public vs. private API

pandas/tests/dtypes/test_dtypes.py

@@ -26,7 +26,14 @@
 )
 
 import pandas as pd
-from pandas import Categorical, CategoricalIndex, IntervalIndex, Series, date_range
+from pandas import (
+    Categorical,
+    CategoricalIndex,
+    DatetimeIndex,
+    IntervalIndex,
+    Series,
+    date_range,
+)
 import pandas._testing as tm
 from pandas.core.arrays.sparse import SparseArray, SparseDtype
 
@@ -177,6 +184,11 @@ def test_is_boolean(self, categories, expected):
         assert is_bool_dtype(cat) is expected
         assert is_bool_dtype(cat.dtype) is expected
 
+    def test_dtype_specific_categorical_dtype(self):
+        expected = "datetime64[ns]"
+        result = str(Categorical(DatetimeIndex([])).categories.dtype)
+        assert result == expected
+
 
 class TestDatetimeTZDtype(Base):
     @pytest.fixture

pandas/tests/frame/test_alter_axes.py

@@ -689,44 +689,6 @@ def test_rename_axis_mapper(self):
         with pytest.raises(TypeError, match="bogus"):
             df.rename_axis(bogus=None)
 
-    def test_reorder_levels(self):
-        index = MultiIndex(
-            levels=[["bar"], ["one", "two", "three"], [0, 1]],
-            codes=[[0, 0, 0, 0, 0, 0], [0, 1, 2, 0, 1, 2], [0, 1, 0, 1, 0, 1]],
-            names=["L0", "L1", "L2"],
-        )
-        df = DataFrame({"A": np.arange(6), "B": np.arange(6)}, index=index)
-
-        # no change, position
-        result = df.reorder_levels([0, 1, 2])
-        tm.assert_frame_equal(df, result)
-
-        # no change, labels
-        result = df.reorder_levels(["L0", "L1", "L2"])
-        tm.assert_frame_equal(df, result)
-
-        # rotate, position
-        result = df.reorder_levels([1, 2, 0])
-        e_idx = MultiIndex(
-            levels=[["one", "two", "three"], [0, 1], ["bar"]],
-            codes=[[0, 1, 2, 0, 1, 2], [0, 1, 0, 1, 0, 1], [0, 0, 0, 0, 0, 0]],
-            names=["L1", "L2", "L0"],
-        )
-        expected = DataFrame({"A": np.arange(6), "B": np.arange(6)}, index=e_idx)
-        tm.assert_frame_equal(result, expected)
-
-        result = df.reorder_levels([0, 0, 0])
-        e_idx = MultiIndex(
-            levels=[["bar"], ["bar"], ["bar"]],
-            codes=[[0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0]],
-            names=["L0", "L0", "L0"],
-        )
-        expected = DataFrame({"A": np.arange(6), "B": np.arange(6)}, index=e_idx)
-        tm.assert_frame_equal(result, expected)
-
-        result = df.reorder_levels(["L0", "L0", "L0"])
-        tm.assert_frame_equal(result, expected)
-
     def test_set_index_names(self):
         df = tm.makeDataFrame()
         df.index.name = "name"

pandas/tests/generic/methods/test_reorder_levels.py

@@ -0,0 +1,73 @@
+import numpy as np
+import pytest
+
+from pandas import DataFrame, MultiIndex, Series
+import pandas._testing as tm
+
+
+class TestReorderLevels:
+    @pytest.mark.parametrize("klass", [Series, DataFrame])
+    def test_reorder_levels(self, klass):
+        index = MultiIndex(
+            levels=[["bar"], ["one", "two", "three"], [0, 1]],
+            codes=[[0, 0, 0, 0, 0, 0], [0, 1, 2, 0, 1, 2], [0, 1, 0, 1, 0, 1]],
+            names=["L0", "L1", "L2"],
+        )
+        df = DataFrame({"A": np.arange(6), "B": np.arange(6)}, index=index)
+        obj = df if klass is DataFrame else df["A"]
+
+        # no change, position
+        result = obj.reorder_levels([0, 1, 2])
+        tm.assert_equal(obj, result)
+
+        # no change, labels
+        result = obj.reorder_levels(["L0", "L1", "L2"])
+        tm.assert_equal(obj, result)
+
+        # rotate, position
+        result = obj.reorder_levels([1, 2, 0])
+        e_idx = MultiIndex(
+            levels=[["one", "two", "three"], [0, 1], ["bar"]],
+            codes=[[0, 1, 2, 0, 1, 2], [0, 1, 0, 1, 0, 1], [0, 0, 0, 0, 0, 0]],
+            names=["L1", "L2", "L0"],
+        )
+        expected = DataFrame({"A": np.arange(6), "B": np.arange(6)}, index=e_idx)
+        expected = expected if klass is DataFrame else expected["A"]
+        tm.assert_equal(result, expected)
+
+        result = obj.reorder_levels([0, 0, 0])
+        e_idx = MultiIndex(
+            levels=[["bar"], ["bar"], ["bar"]],
+            codes=[[0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0]],
+            names=["L0", "L0", "L0"],
+        )
+        expected = DataFrame({"A": np.arange(6), "B": np.arange(6)}, index=e_idx)
+        expected = expected if klass is DataFrame else expected["A"]
+        tm.assert_equal(result, expected)
+
+        result = obj.reorder_levels(["L0", "L0", "L0"])
+        tm.assert_equal(result, expected)
+
+    def test_reorder_levels_swaplevel_equivalence(
+        self, multiindex_year_month_day_dataframe_random_data
+    ):
+
+        ymd = multiindex_year_month_day_dataframe_random_data
+
+        result = ymd.reorder_levels(["month", "day", "year"])
+        expected = ymd.swaplevel(0, 1).swaplevel(1, 2)
+        tm.assert_frame_equal(result, expected)
+
+        result = ymd["A"].reorder_levels(["month", "day", "year"])
+        expected = ymd["A"].swaplevel(0, 1).swaplevel(1, 2)
+        tm.assert_series_equal(result, expected)
+
+        result = ymd.T.reorder_levels(["month", "day", "year"], axis=1)
+        expected = ymd.T.swaplevel(0, 1, axis=1).swaplevel(1, 2, axis=1)
+        tm.assert_frame_equal(result, expected)
+
+        with pytest.raises(TypeError, match="hierarchical axis"):
+            ymd.reorder_levels([1, 2], axis=1)
+
+        with pytest.raises(IndexError, match="Too many levels"):
+            ymd.index.reorder_levels([1, 2, 3])

pandas/tests/indexing/multiindex/conftest.py

@@ -2,7 +2,6 @@
 import pytest
 
 from pandas import DataFrame, Index, MultiIndex
-import pandas._testing as tm
 
 
 @pytest.fixture
@@ -16,17 +15,3 @@ def multiindex_dataframe_random_data():
     return DataFrame(
         np.random.randn(10, 3), index=index, columns=Index(["A", "B", "C"], name="exp")
     )
-
-
-@pytest.fixture
-def multiindex_year_month_day_dataframe_random_data():
-    """
-    DataFrame with 3 level MultiIndex (year, month, day) covering
-    first 100 business days from 2000-01-01 with random data
-    """
-    tdf = tm.makeTimeDataFrame(100)
-    ymd = tdf.groupby([lambda x: x.year, lambda x: x.month, lambda x: x.day]).sum()
-    # use Int64Index, to make sure things work
-    ymd.index.set_levels([lev.astype("i8") for lev in ymd.index.levels], inplace=True)
-    ymd.index.set_names(["year", "month", "day"], inplace=True)
-    return ymd

pandas/tests/series/test_alter_axes.py

@@ -54,32 +54,6 @@ def test_set_index_makes_timeseries(self):
         s.index = idx
         assert s.index.is_all_dates
 
-    def test_reorder_levels(self):
-        index = MultiIndex(
-            levels=[["bar"], ["one", "two", "three"], [0, 1]],
-            codes=[[0, 0, 0, 0, 0, 0], [0, 1, 2, 0, 1, 2], [0, 1, 0, 1, 0, 1]],
-            names=["L0", "L1", "L2"],
-        )
-        s = Series(np.arange(6), index=index)
-
-        # no change, position
-        result = s.reorder_levels([0, 1, 2])
-        tm.assert_series_equal(s, result)
-
-        # no change, labels
-        result = s.reorder_levels(["L0", "L1", "L2"])
-        tm.assert_series_equal(s, result)
-
-        # rotate, position
-        result = s.reorder_levels([1, 2, 0])
-        e_idx = MultiIndex(
-            levels=[["one", "two", "three"], [0, 1], ["bar"]],
-            codes=[[0, 1, 2, 0, 1, 2], [0, 1, 0, 1, 0, 1], [0, 0, 0, 0, 0, 0]],
-            names=["L1", "L2", "L0"],
-        )
-        expected = Series(np.arange(6), index=e_idx)
-        tm.assert_series_equal(result, expected)
-
     def test_rename_axis_mapper(self):
         # GH 19978
         mi = MultiIndex.from_product([["a", "b", "c"], [1, 2]], names=["ll", "nn"])

pandas/tests/test_multilevel.py

@@ -939,25 +939,6 @@ def test_swaplevel(self):
         with pytest.raises(TypeError, match=msg):
             DataFrame(range(3)).swaplevel()
 
-    def test_reorder_levels(self):
-        result = self.ymd.reorder_levels(["month", "day", "year"])
-        expected = self.ymd.swaplevel(0, 1).swaplevel(1, 2)
-        tm.assert_frame_equal(result, expected)
-
-        result = self.ymd["A"].reorder_levels(["month", "day", "year"])
-        expected = self.ymd["A"].swaplevel(0, 1).swaplevel(1, 2)
-        tm.assert_series_equal(result, expected)
-
-        result = self.ymd.T.reorder_levels(["month", "day", "year"], axis=1)
-        expected = self.ymd.T.swaplevel(0, 1, axis=1).swaplevel(1, 2, axis=1)
-        tm.assert_frame_equal(result, expected)
-
-        with pytest.raises(TypeError, match="hierarchical axis"):
-            self.ymd.reorder_levels([1, 2], axis=1)
-
-        with pytest.raises(IndexError, match="Too many levels"):
-            self.ymd.index.reorder_levels([1, 2, 3])
-
     def test_insert_index(self):
         df = self.ymd[:5].T
         df[2000, 1, 10] = df[2000, 1, 7]