BUGfix: reset_index can reset index name (#17067)

doc/source/whatsnew/v0.21.0.txt

@@ -324,6 +324,7 @@ Indexing
 - Fixes ``DataFrame.loc`` for setting with alignment and tz-aware ``DatetimeIndex`` (:issue:`16889`)
 - Avoids ``IndexError`` when passing an Index or Series to ``.iloc`` with older numpy (:issue:`17193`)
 - Allow unicode empty strings as placeholders in multilevel columns in Python 2 (:issue:`17099`)
+- Fixes in ``Index.__add__`` disappearing index name after plus operation (:issue:`17067`)
 
 I/O
 ^^^

pandas/core/common.py

@@ -204,9 +204,9 @@ def is_bool_indexer(key):
     return False
 
 
-def _default_index(n):
+def _default_index(n, name=None):
     from pandas.core.index import RangeIndex
-    return RangeIndex(0, n, name=None)
+    return RangeIndex(0, n, name=name)
 
 
 def _mut_exclusive(**kwargs):

pandas/core/indexes/base.py

@@ -2110,7 +2110,7 @@ def argsort(self, *args, **kwargs):
         return result.argsort(*args, **kwargs)
 
     def __add__(self, other):
-        return Index(np.array(self) + other)
+        return Index(np.array(self) + other, name=self.name)
 
     def __radd__(self, other):
         return Index(other + np.array(self))

pandas/tests/frame/test_alter_axes.py

@@ -802,6 +802,11 @@ def test_reset_index_range(self):
                                 index=RangeIndex(stop=2))
         assert_frame_equal(result, expected)
 
+    def test_reset_index_name(self):
+        # issue `17067`: after reset_index, index name should be None
+        df = pd.DataFrame(index=pd.Index([], name='x'))
+        assert df.reset_index(level=[]).index.name is None
+
     def test_set_index_names(self):
         df = pd.util.testing.makeDataFrame()
         df.index.name = 'name'

pandas/tests/indexes/test_base.py

@@ -859,6 +859,14 @@ def test_iadd_string(self):
         index += '_x'
         assert 'a_x' in index
 
+    def test_iadd_index_name(self):
+        # issue `17067`: test Index keeping index name after plus op
+        s = pd.Series([1, 2, 3])
+        s.index.name = 'foo'
+
+        s.index += 1
+        assert s.index.name == 'foo'
+
     def test_difference(self):
 
         first = self.strIndex[5:20]