refactor: put right import for isna method

Author: Justine Kosinski

pandas/io/formats/excel.py

@@ -628,10 +628,10 @@ def _format_header_mi(self) -> Iterable[ExcelCell]:
             fixed_levels = []
             for lvl in range(columns.nlevels):
                 vals = columns.get_level_values(lvl)
-                fixed_levels.append([NBSP if pd.isna(v) else str(v) for v in vals])
+                fixed_levels.append([NBSP if missing.isna(v) else str(v) for v in vals])
             fixed_columns = MultiIndex.from_arrays(fixed_levels, names=columns.names)
         else:
-            fixed_columns = Index([NBSP if pd.isna(v) else str(v) for v in columns], name=columns.name)
+            fixed_columns = Index([NBSP if missing.isna(v) else str(v) for v in columns], name=columns.name)
 
         level_strs = fixed_columns._format_multi(sparsify=merge_columns, include_names=False)
         level_lengths = get_level_lengths(level_strs)
@@ -642,7 +642,7 @@ def _format_header_mi(self) -> Iterable[ExcelCell]:
             coloffset = self.df.index.nlevels - 1
 
         for lnum, name in enumerate(columns.names):
-            val = NBSP if pd.isna(name) else str(name)
+            val = NBSP if missing.isna(name) else str(name)
             yield ExcelCell(
                 row=lnum,
                 col=coloffset,
@@ -702,7 +702,7 @@ def _format_header_regular(self) -> Iterable[ExcelCell]:
             # header output matches console display (same behavior as
             # applied to MultiIndex headers in _format_header_mi).
             NBSP = "\u00A0"
-            colnames = [NBSP if pd.isna(v) else str(v) for v in colnames]
+            colnames = [NBSP if missing.isna(v) else str(v) for v in colnames]
 
             for colindex, colname in enumerate(colnames):
                 yield CssExcelCell(