BUG: Don't merge Excel cells to a single row with merge_cells=False (#60293)

* BUG: Don't merge Excel cells to a single row with merge_cells=False

* Cleanup comment

* Improve test

Author: rhshadrach

doc/source/whatsnew/v3.0.0.rst

@@ -702,6 +702,7 @@ I/O
 - Bug in :meth:`read_stata` raising ``KeyError`` when input file is stored in big-endian format and contains strL data. (:issue:`58638`)
 - Bug in :meth:`read_stata` where extreme value integers were incorrectly interpreted as missing for format versions 111 and prior (:issue:`58130`)
 - Bug in :meth:`read_stata` where the missing code for double was not recognised for format versions 105 and prior (:issue:`58149`)
+- Bug in :meth:`to_excel` where :class:`MultiIndex` columns would be merged to a single row when ``merge_cells=False`` is passed (:issue:`60274`)
 
 Period
 ^^^^^^

pandas/io/formats/excel.py

@@ -48,7 +48,6 @@
     CSSWarning,
 )
 from pandas.io.formats.format import get_level_lengths
-from pandas.io.formats.printing import pprint_thing
 
 if TYPE_CHECKING:
     from pandas._typing import (
@@ -620,61 +619,43 @@ def _format_header_mi(self) -> Iterable[ExcelCell]:
             return
 
         columns = self.columns
-        level_strs = columns._format_multi(
-            sparsify=self.merge_cells in {True, "columns"}, include_names=False
-        )
+        merge_columns = self.merge_cells in {True, "columns"}
+        level_strs = columns._format_multi(sparsify=merge_columns, include_names=False)
         level_lengths = get_level_lengths(level_strs)
         coloffset = 0
         lnum = 0
 
         if self.index and isinstance(self.df.index, MultiIndex):
             coloffset = self.df.index.nlevels - 1
 
-        if self.merge_cells in {True, "columns"}:
-            # Format multi-index as a merged cells.
-            for lnum, name in enumerate(columns.names):
-                yield ExcelCell(
-                    row=lnum,
-                    col=coloffset,
-                    val=name,
-                    style=None,
-                )
+        for lnum, name in enumerate(columns.names):
+            yield ExcelCell(
+                row=lnum,
+                col=coloffset,
+                val=name,
+                style=None,
+            )
 
-            for lnum, (spans, levels, level_codes) in enumerate(
-                zip(level_lengths, columns.levels, columns.codes)
-            ):
-                values = levels.take(level_codes)
-                for i, span_val in spans.items():
-                    mergestart, mergeend = None, None
-                    if span_val > 1:
-                        mergestart, mergeend = lnum, coloffset + i + span_val
-                    yield CssExcelCell(
-                        row=lnum,
-                        col=coloffset + i + 1,
-                        val=values[i],
-                        style=None,
-                        css_styles=getattr(self.styler, "ctx_columns", None),
-                        css_row=lnum,
-                        css_col=i,
-                        css_converter=self.style_converter,
-                        mergestart=mergestart,
-                        mergeend=mergeend,
-                    )
-        else:
-            # Format in legacy format with dots to indicate levels.
-            for i, values in enumerate(zip(*level_strs)):
-                v = ".".join(map(pprint_thing, values))
+        for lnum, (spans, levels, level_codes) in enumerate(
+            zip(level_lengths, columns.levels, columns.codes)
+        ):
+            values = levels.take(level_codes)
+            for i, span_val in spans.items():
+                mergestart, mergeend = None, None
+                if merge_columns and span_val > 1:
+                    mergestart, mergeend = lnum, coloffset + i + span_val
                 yield CssExcelCell(
                     row=lnum,
                     col=coloffset + i + 1,
-                    val=v,
+                    val=values[i],
                     style=None,
                     css_styles=getattr(self.styler, "ctx_columns", None),
                     css_row=lnum,
                     css_col=i,
                     css_converter=self.style_converter,
+                    mergestart=mergestart,
+                    mergeend=mergeend,
                 )
-
         self.rowcounter = lnum
 
     def _format_header_regular(self) -> Iterable[ExcelCell]:
@@ -798,11 +779,8 @@ def _format_hierarchical_rows(self) -> Iterable[ExcelCell]:
 
             # MultiIndex columns require an extra row
             # with index names (blank if None) for
-            # unambiguous round-trip, unless not merging,
-            # in which case the names all go on one row Issue #11328
-            if isinstance(self.columns, MultiIndex) and (
-                self.merge_cells in {True, "columns"}
-            ):
+            # unambiguous round-trip, Issue #11328
+            if isinstance(self.columns, MultiIndex):
                 self.rowcounter += 1
 
             # if index labels are not empty go ahead and dump

pandas/tests/io/excel/test_writers.py

@@ -870,27 +870,49 @@ def test_to_excel_multiindex_nan_label(self, merge_cells, tmp_excel):
     # Test for Issue 11328. If column indices are integers, make
     # sure they are handled correctly for either setting of
     # merge_cells
-    def test_to_excel_multiindex_cols(self, merge_cells, frame, tmp_excel):
+    def test_to_excel_multiindex_cols(self, merge_cells, tmp_excel):
+        # GH#11328
+        frame = DataFrame(
+            {
+                "A": [1, 2, 3],
+                "B": [4, 5, 6],
+                "C": [7, 8, 9],
+            }
+        )
         arrays = np.arange(len(frame.index) * 2, dtype=np.int64).reshape(2, -1)
         new_index = MultiIndex.from_arrays(arrays, names=["first", "second"])
         frame.index = new_index
 
-        new_cols_index = MultiIndex.from_tuples([(40, 1), (40, 2), (50, 1), (50, 2)])
+        new_cols_index = MultiIndex.from_tuples([(40, 1), (40, 2), (50, 1)])
         frame.columns = new_cols_index
-        header = [0, 1]
-        if not merge_cells:
-            header = 0
-
-        # round trip
         frame.to_excel(tmp_excel, sheet_name="test1", merge_cells=merge_cells)
+
+        # Check round trip
+        with ExcelFile(tmp_excel) as reader:
+            result = pd.read_excel(
+                reader, sheet_name="test1", header=[0, 1], index_col=[0, 1]
+            )
+        tm.assert_frame_equal(result, frame)
+
+        # GH#60274
+        # Check with header/index_col None to determine which cells were merged
         with ExcelFile(tmp_excel) as reader:
-            df = pd.read_excel(
-                reader, sheet_name="test1", header=header, index_col=[0, 1]
+            result = pd.read_excel(
+                reader, sheet_name="test1", header=None, index_col=None
             )
+        expected = DataFrame(
+            {
+                0: [np.nan, np.nan, "first", 0, 1, 2],
+                1: [np.nan, np.nan, "second", 3, 4, 5],
+                2: [40.0, 1.0, np.nan, 1.0, 2.0, 3.0],
+                3: [np.nan, 2.0, np.nan, 4.0, 5.0, 6.0],
+                4: [50.0, 1.0, np.nan, 7.0, 8.0, 9.0],
+            }
+        )
         if not merge_cells:
-            fm = frame.columns._format_multi(sparsify=False, include_names=False)
-            frame.columns = [".".join(map(str, q)) for q in zip(*fm)]
-        tm.assert_frame_equal(frame, df)
+            # MultiIndex column value is repeated
+            expected.loc[0, 3] = 40.0
+        tm.assert_frame_equal(result, expected)
 
     def test_to_excel_multiindex_dates(self, merge_cells, tmp_excel):
         # try multiindex with dates