GH804 Fix property setter/getter for Index.names

Author: loicdiridollou

pandas-stubs/core/indexes/base.pyi

@@ -55,6 +55,7 @@ from pandas._typing import (
     MaskType,
     NaPosition,
     ReindexMethod,
+    SequenceNotStr,
     SliceType,
     TimedeltaDtypeArg,
     TimestampDtypeArg,
@@ -306,13 +307,13 @@ class Index(IndexOpsMixin[S1]):
     def to_series(self, index=..., name: Hashable = ...) -> Series: ...
     def to_frame(self, index: bool = ..., name=...) -> DataFrame: ...
     @property
-    def name(self) -> Hashable | None: ...
+    def name(self) -> _str | None: ...
     @name.setter
     def name(self, value) -> None: ...
     @property
-    def names(self) -> list[Hashable]: ...
+    def names(self) -> list[_str | None]: ...
     @names.setter
-    def names(self, names: Sequence[Hashable]) -> None: ...
+    def names(self, names: SequenceNotStr[_str | None]) -> None: ...
     def set_names(self, names, *, level=..., inplace: bool = ...): ...
     @overload
     def rename(self, name, *, inplace: Literal[False] = False) -> Self: ...

tests/test_indexes.py

@@ -1435,3 +1435,21 @@ def test_multiindex_range() -> None:
         [range(3), pd.Series([2, 3, 5])],
     )
     check(assert_type(midx_mixed_types, pd.MultiIndex), pd.MultiIndex)
+
+
+def test_index_naming() -> None:
+    """
+    Test index names type both for the getter and the setter.
+    The names of an index should be settable with a sequence (not str) and names
+    property is a list[str | None] (FrozenList).
+    """
+    df = pd.DataFrame({"a": ["a", "b", "c"], "i": [10, 11, 12]})
+
+    df.index.names = ["idx"]
+    check(assert_type(df.index.names, list[str | None]), list)
+    df.index.names = ("idx2",)
+    check(assert_type(df.index.names, list[str | None]), list)
+    df.index.names = [None]
+    check(assert_type(df.index.names, list[str | None]), list)
+    df.index.names = (None,)
+    check(assert_type(df.index.names, list[str | None]), list)