TYP: define gen and gen2 as Iterable

Author: Kashif Khan

pandas/io/formats/excel.py

@@ -649,12 +649,15 @@ def _format_header_regular(self) -> Iterable[ExcelCell]:
                 )
 
     def _format_header(self) -> Iterable[ExcelCell]:
+        gen: Iterable[ExcelCell]
+
         if isinstance(self.columns, MultiIndex):
             gen = self._format_header_mi()
         else:
             gen = self._format_header_regular()
 
-        gen2 = ()
+        gen2: Iterable[ExcelCell] | tuple[()] = ()
+
         if self.df.index.names:
             row = [x if x is not None else "" for x in self.df.index.names] + [
                 ""