API: Restore getting name from MultiIndex level (#29061)

* API: Restore getting name from MultiIndex level

xref https://issues.apache.org/jira/browse/ARROW-6922 /
#27242 (comment)
/ #29032

No docs yet, since it isn't clear how this will eventually sort out. But
we at least want to preserve this behavior for 1.0

* fixups

pandas/core/indexes/multi.py

@@ -639,7 +639,10 @@ def from_frame(cls, df, sortorder=None, names=None):
 
     @property
     def levels(self):
-        return self._levels
+        result = [
+            x._shallow_copy(name=name) for x, name in zip(self._levels, self._names)
+        ]
+        return FrozenList(result)
 
     @property
     def _values(self):
@@ -830,7 +833,7 @@ def _set_codes(
         if level is None:
             new_codes = FrozenList(
                 _ensure_frozen(level_codes, lev, copy=copy)._shallow_copy()
-                for lev, level_codes in zip(self.levels, codes)
+                for lev, level_codes in zip(self._levels, codes)
             )
         else:
             level = [self._get_level_number(l) for l in level]

pandas/tests/frame/test_alter_axes.py

@@ -978,7 +978,7 @@ def test_reset_index(self, float_frame):
         ):
             values = lev.take(level_codes)
             name = names[i]
-            tm.assert_index_equal(values, Index(deleveled[name].rename(name=None)))
+            tm.assert_index_equal(values, Index(deleveled[name]))
 
         stacked.index.names = [None, None]
         deleveled2 = stacked.reset_index()

pandas/tests/indexes/multi/test_constructor.py

@@ -17,7 +17,7 @@ def test_constructor_single_level():
         levels=[["foo", "bar", "baz", "qux"]], codes=[[0, 1, 2, 3]], names=["first"]
     )
     assert isinstance(result, MultiIndex)
-    expected = Index(["foo", "bar", "baz", "qux"])
+    expected = Index(["foo", "bar", "baz", "qux"], name="first")
     tm.assert_index_equal(result.levels[0], expected)
     assert result.names == ["first"]
 
@@ -292,7 +292,7 @@ def test_from_arrays_empty():
     # 1 level
     result = MultiIndex.from_arrays(arrays=[[]], names=["A"])
     assert isinstance(result, MultiIndex)
-    expected = Index([])
+    expected = Index([], name="A")
     tm.assert_index_equal(result.levels[0], expected)
     assert result.names == ["A"]
 
@@ -440,7 +440,7 @@ def test_from_product_empty_zero_levels():
 
 def test_from_product_empty_one_level():
     result = MultiIndex.from_product([[]], names=["A"])
-    expected = pd.Index([])
+    expected = pd.Index([], name="A")
     tm.assert_index_equal(result.levels[0], expected)
     assert result.names == ["A"]
 

pandas/tests/indexes/multi/test_names.py

@@ -27,7 +27,7 @@ def test_index_name_retained():
 
 
 def test_changing_names(idx):
-    assert [level.name for level in idx.levels] == [None, None]
+    assert [level.name for level in idx.levels] == ["first", "second"]
 
     view = idx.view()
     copy = idx.copy()
@@ -36,16 +36,16 @@ def test_changing_names(idx):
     # changing names should not change level names on object
     new_names = [name + "a" for name in idx.names]
     idx.names = new_names
-    check_level_names(idx, [None, None])
+    check_level_names(idx, ["firsta", "seconda"])
 
     # and not on copies
-    check_level_names(view, [None, None])
-    check_level_names(copy, [None, None])
-    check_level_names(shallow_copy, [None, None])
+    check_level_names(view, ["first", "second"])
+    check_level_names(copy, ["first", "second"])
+    check_level_names(shallow_copy, ["first", "second"])
 
     # and copies shouldn't change original
     shallow_copy.names = [name + "c" for name in shallow_copy.names]
-    check_level_names(idx, [None, None])
+    check_level_names(idx, ["firsta", "seconda"])
 
 
 def test_take_preserve_name(idx):
@@ -81,7 +81,7 @@ def test_names(idx, index_names):
     # names are assigned in setup
     assert index_names == ["first", "second"]
     level_names = [level.name for level in idx.levels]
-    assert level_names == [None, None]
+    assert level_names == index_names
 
     # setting bad names on existing
     index = idx
@@ -109,11 +109,18 @@ def test_names(idx, index_names):
     # names are assigned on index, but not transferred to the levels
     index.names = ["a", "b"]
     level_names = [level.name for level in index.levels]
-    assert level_names == [None, None]
+    assert level_names == ["a", "b"]
 
 
 def test_duplicate_level_names_access_raises(idx):
     # GH19029
     idx.names = ["foo", "foo"]
     with pytest.raises(ValueError, match="name foo occurs multiple times"):
         idx._get_level_number("foo")
+
+
+def test_get_names_from_levels():
+    idx = pd.MultiIndex.from_product([["a"], [1, 2]], names=["a", "b"])
+
+    assert idx.levels[0].name == "a"
+    assert idx.levels[1].name == "b"

pandas/tests/indexes/multi/test_reindex.py

@@ -10,13 +10,13 @@ def test_reindex(idx):
     result, indexer = idx.reindex(list(idx[:4]))
     assert isinstance(result, MultiIndex)
     assert result.names == ["first", "second"]
-    assert [level.name for level in result.levels] == [None, None]
+    assert [level.name for level in result.levels] == ["first", "second"]
 
     result, indexer = idx.reindex(list(idx))
     assert isinstance(result, MultiIndex)
     assert indexer is None
     assert result.names == ["first", "second"]
-    assert [level.name for level in result.levels] == [None, None]
+    assert [level.name for level in result.levels] == ["first", "second"]
 
 
 def test_reindex_level(idx):

pandas/tests/indexes/multi/test_reshape.py

@@ -15,11 +15,11 @@ def test_insert(idx):
     # key not contained in all levels
     new_index = idx.insert(0, ("abc", "three"))
 
-    exp0 = Index(list(idx.levels[0]) + ["abc"])
+    exp0 = Index(list(idx.levels[0]) + ["abc"], name="first")
     tm.assert_index_equal(new_index.levels[0], exp0)
     assert new_index.names == ["first", "second"]
 
-    exp1 = Index(list(idx.levels[1]) + ["three"])
+    exp1 = Index(list(idx.levels[1]) + ["three"], name="second")
     tm.assert_index_equal(new_index.levels[1], exp1)
     assert new_index[0] == ("abc", "three")
 

pandas/tests/reshape/test_concat.py

@@ -1219,7 +1219,7 @@ def test_concat_keys_specific_levels(self):
             names=["group_key"],
         )
 
-        tm.assert_index_equal(result.columns.levels[0], Index(level))
+        tm.assert_index_equal(result.columns.levels[0], Index(level, name="group_key"))
         tm.assert_index_equal(result.columns.levels[1], Index([0, 1, 2, 3]))
 
         assert result.columns.names == ["group_key", None]
@@ -1412,7 +1412,9 @@ def test_concat_keys_and_levels(self):
             names=["first", "second"],
         )
         assert result.index.names == ("first", "second", None)
-        tm.assert_index_equal(result.index.levels[0], Index(["baz", "foo"]))
+        tm.assert_index_equal(
+            result.index.levels[0], Index(["baz", "foo"], name="first")
+        )
 
     def test_concat_keys_levels_no_overlap(self):
         # GH #1406

pandas/tests/test_multilevel.py

@@ -975,11 +975,11 @@ def test_count(self):
         series.index.names = ["a", "b"]
 
         result = series.count(level="b")
-        expect = self.series.count(level=1)
+        expect = self.series.count(level=1).rename_axis("b")
         tm.assert_series_equal(result, expect)
 
         result = series.count(level="a")
-        expect = self.series.count(level=0)
+        expect = self.series.count(level=0).rename_axis("a")
         tm.assert_series_equal(result, expect)
 
         msg = "Level x not found"
@@ -1641,16 +1641,14 @@ def test_constructor_with_tz(self):
         result = MultiIndex.from_arrays([index, columns])
 
         assert result.names == ["dt1", "dt2"]
-        # levels don't have names set, so set name of index/columns to None in checks
-        tm.assert_index_equal(result.levels[0], index.rename(name=None))
-        tm.assert_index_equal(result.levels[1], columns.rename(name=None))
+        tm.assert_index_equal(result.levels[0], index)
+        tm.assert_index_equal(result.levels[1], columns)
 
         result = MultiIndex.from_arrays([Series(index), Series(columns)])
 
         assert result.names == ["dt1", "dt2"]
-        # levels don't have names set, so set name of index/columns to None in checks
-        tm.assert_index_equal(result.levels[0], index.rename(name=None))
-        tm.assert_index_equal(result.levels[1], columns.rename(name=None))
+        tm.assert_index_equal(result.levels[0], index)
+        tm.assert_index_equal(result.levels[1], columns)
 
     def test_set_index_datetime(self):
         # GH 3950
@@ -1672,17 +1670,18 @@ def test_set_index_datetime(self):
         df.index = df.index.tz_convert("US/Pacific")
 
         expected = pd.DatetimeIndex(
-            ["2011-07-19 07:00:00", "2011-07-19 08:00:00", "2011-07-19 09:00:00"]
+            ["2011-07-19 07:00:00", "2011-07-19 08:00:00", "2011-07-19 09:00:00"],
+            name="datetime",
         )
         expected = expected.tz_localize("UTC").tz_convert("US/Pacific")
 
         df = df.set_index("label", append=True)
         tm.assert_index_equal(df.index.levels[0], expected)
-        tm.assert_index_equal(df.index.levels[1], Index(["a", "b"]))
+        tm.assert_index_equal(df.index.levels[1], Index(["a", "b"], name="label"))
         assert df.index.names == ["datetime", "label"]
 
         df = df.swaplevel(0, 1)
-        tm.assert_index_equal(df.index.levels[0], Index(["a", "b"]))
+        tm.assert_index_equal(df.index.levels[0], Index(["a", "b"], name="label"))
         tm.assert_index_equal(df.index.levels[1], expected)
         assert df.index.names == ["label", "datetime"]