ENH: Raise error on trying to write excel file with a MultiIndexed DataFrame. closes #9794

Author: hsperr

doc/source/whatsnew/v0.16.1.txt

@@ -52,6 +52,8 @@ Enhancements
 
 - Allow timedelta string conversion when leading zero is missing from time definition, ie `0:00:00` vs `00:00:00`. (:issue:`9570`)
 
+- Trying to write an excel file now raises error if the DataFrame has a MultiIndex instead of writing a broken Excel file. (:issue:`9794`)
+
 .. _whatsnew_0161.api:
 
 API changes
@@ -132,3 +134,4 @@ Bug Fixes
 - Bug in unequal comparisons between a ``Series`` of dtype `"category"` and a scalar (e.g. ``Series(Categorical(list("abc"), categories=list("cba"), ordered=True)) > "b"``, which wouldn't use the order of the categories but use the lexicographical order. (:issue:`9848`)
 
 - Bug in unequal comparisons between categorical data and a scalar, which was not in the categories (e.g. ``Series(Categorical(list("abc"), ordered=True)) > "d"``. This returned ``False`` for all elements, but now raises a ``TypeError``. Equality comparisons also now return ``False`` for ``==`` and ``True`` for ``!=``. (:issue:`9848`)
+

pandas/core/frame.py

@@ -1244,6 +1244,10 @@ def to_excel(self, excel_writer, sheet_name='Sheet1', na_rep='',
         >>> writer.save()
         """
         from pandas.io.excel import ExcelWriter
+        print self.index.nlevels
+        if self.index.nlevels > 1 or self.columns.nlevels > 1:
+            raise NotImplementedError("Writing as Excel with a MultiIndex is "
+                                      "not yet implemented.")
 
         need_save = False
         if encoding == None: