BUG: Fix default encoding for CSVFormatter.save

Author: Licht-T

pandas/io/formats/format.py

@@ -1573,12 +1573,20 @@ def __init__(self, obj, path_or_buf=None, sep=",", na_rep='',
 
     def save(self):
         # create the writer & save
+        if self.encoding is None:
+            if compat.PY2:
+                encoding = 'ascii'
+            else:
+                encoding = 'utf-8'
+        else:
+            encoding = self.encoding
+
         if hasattr(self.path_or_buf, 'write'):
             f = self.path_or_buf
             close = False
         else:
             f, handles = _get_handle(self.path_or_buf, self.mode,
-                                     encoding=self.encoding,
+                                     encoding=encoding,
                                      compression=self.compression)
             close = True
 
@@ -1588,11 +1596,11 @@ def save(self):
                                  doublequote=self.doublequote,
                                  escapechar=self.escapechar,
                                  quotechar=self.quotechar)
-            if self.encoding is not None:
+            if encoding is 'ascii':
+                self.writer = csv.writer(f, **writer_kwargs)
+            else:
                 writer_kwargs['encoding'] = self.encoding
                 self.writer = UnicodeWriter(f, **writer_kwargs)
-            else:
-                self.writer = csv.writer(f, **writer_kwargs)
 
             self._save()