BUG: XlsxWriter ignoring formats on numpy types if merged cells (#27006)

Author: riptusk331

doc/source/whatsnew/v0.25.0.rst

@@ -785,6 +785,7 @@ I/O
 - Fixed bug in :func:`pandas.read_csv` where a BOM would result in incorrect parsing using engine='python' (:issue:`26545`)
 - :func:`read_excel` now raises a ``ValueError`` when input is of type :class:`pandas.io.excel.ExcelFile` and ``engine`` param is passed since :class:`pandas.io.excel.ExcelFile` has an engine defined (:issue:`26566`)
 - Bug while selecting from :class:`HDFStore` with ``where=''`` specified (:issue:`26610`).
+- Fixed bug in :func:`DataFrame.to_excel()` where custom objects (i.e. `PeriodIndex`) inside merged cells were not being converted into types safe for the Excel writer (:issue:`27006`)
 
 Plotting
 ^^^^^^^^

pandas/io/excel/_xlsxwriter.py

@@ -210,7 +210,7 @@ def write_cells(self, cells, sheet_name=None, startrow=0, startcol=0,
                                 startcol + cell.col,
                                 startrow + cell.mergestart,
                                 startcol + cell.mergeend,
-                                cell.val, style)
+                                val, style)
             else:
                 wks.write(startrow + cell.row,
                           startcol + cell.col,

pandas/tests/io/excel/test_writers.py

@@ -1162,6 +1162,22 @@ def test_path_local_path(self, engine, ext):
                                        path="foo.{ext}".format(ext=ext))
         tm.assert_frame_equal(result, df)
 
+    def test_merged_cell_custom_objects(self, engine, merge_cells, ext):
+        # see GH-27006
+        mi = MultiIndex.from_tuples([(pd.Period('2018'), pd.Period('2018Q1')),
+                                     (pd.Period('2018'), pd.Period('2018Q2'))])
+        expected = DataFrame(np.ones((2, 2)), columns=mi)
+        expected.to_excel(self.path)
+        result = pd.read_excel(self.path, header=[0, 1],
+                               index_col=0, convert_float=False)
+        # need to convert PeriodIndexes to standard Indexes for assert equal
+        expected.columns.set_levels([[str(i) for i in mi.levels[0]],
+                                     [str(i) for i in mi.levels[1]]],
+                                    level=[0, 1],
+                                    inplace=True)
+        expected.index = expected.index.astype(np.float64)
+        tm.assert_frame_equal(expected, result)
+
 
 class TestExcelWriterEngineTests: