API: Raise when setting name via level (#30574)

Author: TomAugspurger

doc/source/user_guide/advanced.rst

@@ -565,19 +565,15 @@ When working with an ``Index`` object directly, rather than via a ``DataFrame``,
  mi2 = mi.rename("new name", level=0)
  mi2
 
-.. warning::
 
- Prior to pandas 1.0.0, you could also set the names of a ``MultiIndex``
- by updating the name of a level.
+You cannot set the names of the MultiIndex via a level.
 
- .. code-block:: none
+.. ipython:: python
+ :okexcept:
 
- >>> mi.levels[0].name = 'name via level'
- >>> mi.names[0] # only works for older pandas
- 'name via level'
+ mi.levels[0].name = "name via level"
 
- As of pandas 1.0, this will *silently* fail to update the names
- of the MultiIndex. Use :meth:`Index.set_names` instead.
+Use :meth:`Index.set_names` instead.
 
 Sorting a ``MultiIndex``
 ------------------------

doc/source/whatsnew/v1.0.0.rst

@@ -260,10 +260,10 @@ For backwards compatibility, you can still *access* the names via the levels.
  mi.levels[0].name
 
 However, it is no longer possible to *update* the names of the ``MultiIndex``
-via the name of the level. The following will **silently** fail to update the
-name of the ``MultiIndex``
+via the level.
 
 .. ipython:: python
+ :okexcept:
 
  mi.levels[0].name = "new name"
  mi.names

pandas/core/indexes/base.py

@@ -240,6 +240,10 @@ def _outer_indexer(self, left, right):
  _data: Union[ExtensionArray, np.ndarray]
  _id = None
  _name: Optional[Hashable] = None
+ # MultiIndex.levels previously allowed setting the index name. We
+ # don't allow this anymore, and raise if it happens rather than
+ # failing silently.
+ _no_setting_name: bool = False
  _comparables = ["name"]
  _attributes = ["name"]
  _is_numeric_dtype = False
@@ -1150,6 +1154,12 @@ def name(self):
 
  @name.setter
  def name(self, value):
+ if self._no_setting_name:
+ # Used in MultiIndex.levels to avoid silently ignoring name updates.
+ raise RuntimeError(
+ "Cannot set name on a level of a MultiIndex. Use "
+ "'MultiIndex.set_names' instead."
+ )
  maybe_extract_name(value, None, type(self))
  self._name = value
 

pandas/core/indexes/category.py

@@ -278,6 +278,7 @@ def _simple_new(cls, values, name=None, dtype=None, **kwargs):
  setattr(result, k, v)
 
  result._reset_identity()
+ result._no_setting_name = False
  return result
 
  # --------------------------------------------------------------------

pandas/core/indexes/datetimes.py

@@ -295,6 +295,7 @@ def _simple_new(cls, values, name=None, freq=None, tz=None, dtype=None):
  result = object.__new__(cls)
  result._data = dtarr
  result.name = name
+ result._no_setting_name = False
  # For groupby perf. See note in indexes/base about _index_data
  result._index_data = dtarr._data
  result._reset_identity()

pandas/core/indexes/interval.py

@@ -261,6 +261,7 @@ def _simple_new(cls, array, name, closed=None):
  result = IntervalMixin.__new__(cls)
  result._data = array
  result.name = name
+ result._no_setting_name = False
  result._reset_identity()
  return result
 

pandas/core/indexes/multi.py

@@ -627,6 +627,9 @@ def levels(self):
  result = [
  x._shallow_copy(name=name) for x, name in zip(self._levels, self._names)
  ]
+ for level in result:
+ # disallow midx.levels[0].name = "foo"
+ level._no_setting_name = True
  return FrozenList(result)
 
  @property

pandas/tests/indexes/multi/test_names.py

@@ -124,3 +124,20 @@ def test_get_names_from_levels():
 
  assert idx.levels[0].name == "a"
  assert idx.levels[1].name == "b"
+
+
+def test_setting_names_from_levels_raises():
+ idx = pd.MultiIndex.from_product([["a"], [1, 2]], names=["a", "b"])
+ with pytest.raises(RuntimeError, match="set_names"):
+ idx.levels[0].name = "foo"
+
+ with pytest.raises(RuntimeError, match="set_names"):
+ idx.levels[1].name = "foo"
+
+ new = pd.Series(1, index=idx.levels[0])
+ with pytest.raises(RuntimeError, match="set_names"):
+ new.index.name = "bar"
+
+ assert pd.Index._no_setting_name is False
+ assert pd.Int64Index._no_setting_name is False
+ assert pd.RangeIndex._no_setting_name is False