DEPR: Deprecate tupleize_cols in to_csv (#17877)

doc/source/whatsnew/v0.21.0.txt

@@ -803,6 +803,7 @@ Deprecations
 - :func:`read_excel()` has deprecated ``sheetname`` in favor of ``sheet_name`` for consistency with ``.to_excel()`` (:issue:`10559`).
 - :func:`read_excel()` has deprecated ``parse_cols`` in favor of ``usecols`` for consistency with :func:`read_csv` (:issue:`4988`)
 - :func:`read_csv()` has deprecated the ``tupleize_cols`` argument. Column tuples will always be converted to a ``MultiIndex`` (:issue:`17060`)
+- :meth:`DataFrame.to_csv` has deprecated the ``tupleize_cols`` argument. Multi-index columns will be always written as rows in the CSV file (:issue:`17060`)
 - The ``convert`` parameter has been deprecated in the ``.take()`` method, as it was not being respected (:issue:`16948`)
 - ``pd.options.html.border`` has been deprecated in favor of ``pd.options.display.html.border`` (:issue:`15793`).
 - :func:`SeriesGroupBy.nth` has deprecated ``True`` in favor of ``'all'`` for its kwarg ``dropna`` (:issue:`11038`).

pandas/core/frame.py

@@ -1432,7 +1432,7 @@ def to_csv(self, path_or_buf=None, sep=",", na_rep='', float_format=None,
                columns=None, header=True, index=True, index_label=None,
                mode='w', encoding=None, compression=None, quoting=None,
                quotechar='"', line_terminator='\n', chunksize=None,
-               tupleize_cols=False, date_format=None, doublequote=True,
+               tupleize_cols=None, date_format=None, doublequote=True,
                escapechar=None, decimal='.'):
         r"""Write DataFrame to a comma-separated values (csv) file
 
@@ -1485,15 +1485,28 @@ def to_csv(self, path_or_buf=None, sep=",", na_rep='', float_format=None,
         chunksize : int or None
             rows to write at a time
         tupleize_cols : boolean, default False
-            write multi_index columns as a list of tuples (if True)
-            or new (expanded format) if False)
+            .. deprecated:: 0.21.0
+               This argument will be removed and will always write each row
+               of the multi-index as a separate row in the CSV file.
+
+            Write MultiIndex columns as a list of tuples (if True) or in
+            the new, expanded format, where each MultiIndex column is a row
+            in the CSV (if False).
         date_format : string, default None
             Format string for datetime objects
         decimal: string, default '.'
             Character recognized as decimal separator. E.g. use ',' for
             European data
 
         """
+
+        if tupleize_cols is not None:
+            warnings.warn("The 'tupleize_cols' parameter is deprecated and "
+                          "will be removed in a future version",
+                          FutureWarning, stacklevel=2)
+        else:
+            tupleize_cols = False
+
         formatter = fmt.CSVFormatter(self, path_or_buf,
                                      line_terminator=line_terminator, sep=sep,
                                      encoding=encoding,

pandas/tests/frame/test_to_csv.py

@@ -577,7 +577,8 @@ def _make_frame(names=None):
 
             # tupleize_cols=True and index=False
             df = _make_frame(True)
-            df.to_csv(path, tupleize_cols=True, index=False)
+            with tm.assert_produces_warning(FutureWarning):
+                df.to_csv(path, tupleize_cols=True, index=False)
 
             with tm.assert_produces_warning(FutureWarning,
                                             check_stacklevel=False):
@@ -602,7 +603,8 @@ def _make_frame(names=None):
 
             # column & index are multi-index (compatibility)
             df = mkdf(5, 3, r_idx_nlevels=2, c_idx_nlevels=4)
-            df.to_csv(path, tupleize_cols=True)
+            with tm.assert_produces_warning(FutureWarning):
+                df.to_csv(path, tupleize_cols=True)
 
             with tm.assert_produces_warning(FutureWarning,
                                             check_stacklevel=False):