BUG: Index.to_series() is not copying the index

Author: jreback

doc/source/whatsnew/v0.20.0.txt

@@ -1148,6 +1148,7 @@ Conversion
 - Bug in ``DataFrame`` construction with nulls and datetimes in a list-like (:issue:`15869`)
 - Bug in ``DataFrame.fillna()`` with tz-aware datetimes (:issue:`15855`)
 - Bug in ``is_string_dtype``, ``is_timedelta64_ns_dtype``, and ``is_string_like_dtype`` in which an error was raised when ``None`` was passed in (:issue:`15941`)
+- Bug in ``Index.to_series()`` where the index was not copied (and so mutating later would change the original), (:issue:`15949`)
 
 Indexing
 ^^^^^^^^

pandas/indexes/base.py

@@ -944,7 +944,9 @@ def to_series(self, **kwargs):
         """
 
         from pandas import Series
-        return Series(self._to_embed(), index=self, name=self.name)
+        return Series(self._to_embed(),
+                      index=self._shallow_copy(),
+                      name=self.name)
 
     def _to_embed(self, keep_tz=False):
         """

pandas/tests/frame/test_query_eval.py

@@ -484,7 +484,7 @@ def test_date_index_query_with_NaT_duplicates(self):
         df = DataFrame(d)
         df.loc[np.random.rand(n) > 0.5, 'dates1'] = pd.NaT
         df.set_index('dates1', inplace=True, drop=True)
-        res = df.query('index < 20130101 < dates3', engine=engine,
+        res = df.query('dates1 < 20130101 < dates3', engine=engine,
                        parser=parser)
         expec = df[(df.index.to_series() < '20130101') &
                    ('20130101' < df.dates3)]

pandas/tests/indexes/common.py

@@ -38,6 +38,15 @@ def test_pickle_compat_construction(self):
         # need an object to create with
         self.assertRaises(TypeError, self._holder)
 
+    def test_to_series(self):
+        # assert that we are creating a copy of the index
+
+        idx = self.create_index()
+        s = idx.to_series()
+        assert s.values is not idx.values
+        assert s.index is not idx
+        assert s.name == idx.name
+
     def test_shift(self):
 
         # GH8083 test the base class for shift

pandas/tseries/index.py

@@ -895,7 +895,9 @@ def to_series(self, keep_tz=False):
         Series
         """
         from pandas import Series
-        return Series(self._to_embed(keep_tz), index=self, name=self.name)
+        return Series(self._to_embed(keep_tz),
+                      index=self._shallow_copy(),
+                      name=self.name)
 
     def _to_embed(self, keep_tz=False):
         """