Fix detection to raise warning

Author: jorisvandenbossche

pandas/io/excel.py

@@ -336,7 +336,7 @@ def _parse_excel(self, sheetname=0, header=0, skiprows=None, names=None,
             raise NotImplementedError("chunksize keyword of read_excel "
                                       "is not implemented")
 
-        if parse_dates and not index_col:
+        if parse_dates is True and not index_col:
             warn("The 'parse_dates=True' keyword of read_excel was provided"
                  " without an 'index_col' keyword value.")
 

pandas/io/tests/test_excel.py

@@ -889,8 +889,9 @@ def test_read_excel_chunksize(self):
     def test_read_excel_parse_dates(self):
         # GH 11544, 12051
 
-        df = DataFrame({'col': [1, 2, 3],
-                        'date_strings': pd.date_range('2012-01-01', periods=3)})
+        df = DataFrame(
+            {'col': [1, 2, 3],
+             'date_strings': pd.date_range('2012-01-01', periods=3)})
         df2 = df.copy()
         df2['date_strings'] = df2['date_strings'].dt.strftime('%m/%d/%Y')