API: Series.to_csv() should return string when path is None

Test cases for when path is None

Return string when path is None

Add note to release notes under API

Add comments to test cases

Author: onesandzeroes

doc/source/v0.15.0.txt

@@ -309,6 +309,9 @@ API changes
      df
      df.dtypes
 
+- ``Series.to_csv()`` now returns a string when ``path=None``, matching the behaviour of
+    ``DataFrame.to_csv()`` (:issue:`8215`).
+
 .. _whatsnew_0150.dt:
 
 .dt accessor

pandas/core/series.py

@@ -2200,7 +2200,8 @@ def to_csv(self, path, index=True, sep=",", na_rep='',
 
         Parameters
         ----------
-        path : string file path or file handle / StringIO
+        path : string file path or file handle / StringIO. If None is provided
+            the result is returned as a string.
         na_rep : string, default ''
             Missing data representation
         float_format : string, default None
@@ -2224,10 +2225,13 @@ def to_csv(self, path, index=True, sep=",", na_rep='',
         """
         from pandas.core.frame import DataFrame
         df = DataFrame(self)
-        df.to_csv(path, index=index, sep=sep, na_rep=na_rep,
+        # result is only a string if no path provided, otherwise None
+        result = df.to_csv(path, index=index, sep=sep, na_rep=na_rep,
                   float_format=float_format, header=header,
                   index_label=index_label, mode=mode, nanRep=nanRep,
                   encoding=encoding, date_format=date_format)
+        if path is None:
+            return result
 
     def dropna(self, axis=0, inplace=False, **kwargs):
         """

pandas/tests/test_frame.py

@@ -6428,6 +6428,14 @@ def test_to_csv_from_csv_categorical(self):
         df2.to_csv(exp)
         self.assertEqual(res.getvalue(), exp.getvalue())
 
+    def test_to_csv_path_is_none(self):
+        # GH 8215
+        # Make sure we return string for consistency with
+        # Series.to_csv()
+        csv_str = self.frame.to_csv(path=None)
+        self.assertIsInstance(csv_str, str)
+        recons = pd.read_csv(StringIO(csv_str), index_col=0)
+        assert_frame_equal(self.frame, recons)
 
     def test_info(self):
         io = StringIO()

pandas/tests/test_series.py

@@ -4371,6 +4371,14 @@ def test_to_csv_list_entries(self):
         buf = StringIO()
         split.to_csv(buf)
 
+    def test_to_csv_path_is_none(self):
+        # GH 8215
+        # Series.to_csv() was returning None, inconsistent with
+        # DataFrame.to_csv() which returned string
+        s = Series([1, 2, 3])
+        csv_str = s.to_csv(path=None)
+        self.assertIsInstance(csv_str, str)
+
     def test_clip(self):
         val = self.ts.median()