DOC: Add comments on to_csv defualt encoding test

Author: Licht-T

pandas/tests/io/formats/test_to_csv.py

@@ -13,7 +13,10 @@ def test_to_csv_defualt_encoding(self):
         df = DataFrame({'col': [u"AAAAA", u"ÄÄÄÄÄ", u"ßßßßß", u"聞聞聞聞聞"]})
 
         with tm.ensure_clean('test.csv') as path:
+            # the default to_csv encoding in Python 2 is ascii, and that in
+            # Python 3 is uft-8.
             if pd.compat.PY2:
+                # the encoding argument parameter should be utf-8
                 with tm.assert_raises_regex(UnicodeEncodeError, 'ascii'):
                     df.to_csv(path)
             else: