TYP: alias IndexLabel without Optional (#36401)

Author: ivanovmg

pandas/_typing.py

@@ -83,7 +83,7 @@
 
 Axis = Union[str, int]
 Label = Optional[Hashable]
-IndexLabel = Optional[Union[Label, Sequence[Label]]]
+IndexLabel = Union[Label, Sequence[Label]]
 Level = Union[Label, int]
 Ordered = Optional[bool]
 JSONSerializable = Optional[Union[PythonScalar, List, Dict]]

pandas/core/generic.py

@@ -3154,7 +3154,7 @@ def to_csv(
         columns: Optional[Sequence[Label]] = None,
         header: Union[bool_t, List[str]] = True,
         index: bool_t = True,
-        index_label: IndexLabel = None,
+        index_label: Optional[IndexLabel] = None,
         mode: str = "w",
         encoding: Optional[str] = None,
         compression: CompressionOptions = "infer",

pandas/io/formats/csvs.py

@@ -42,7 +42,7 @@ def __init__(
         cols: Optional[Sequence[Label]] = None,
         header: Union[bool, Sequence[Hashable]] = True,
         index: bool = True,
-        index_label: IndexLabel = None,
+        index_label: Optional[IndexLabel] = None,
         mode: str = "w",
         encoding: Optional[str] = None,
         errors: str = "strict",
@@ -100,7 +100,7 @@ def index_label(self) -> IndexLabel:
         return self._index_label
 
     @index_label.setter
-    def index_label(self, index_label: IndexLabel) -> None:
+    def index_label(self, index_label: Optional[IndexLabel]) -> None:
         if index_label is not False:
             if index_label is None:
                 index_label = self._get_index_label_from_obj()