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BUG: fix for index name lost #9857 and also added the corresponding test in test_index.py #9876

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BUG: fix for index name lost #9857 and also added the corresponding test in test_index.py #9876

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4 changes: 3 additions & 1 deletion pandas/core/index.py
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Original file line number Diff line number Diff line change
Expand Up @@ -976,7 +976,9 @@ def append(self, other):
to_concat.append(other)

for obj in to_concat:
if isinstance(obj, Index) and obj.name != name:
if (isinstance(obj, Index) and
obj.name != name and
obj.name is not None):
name = None
break

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13 changes: 13 additions & 0 deletions pandas/tests/test_index.py
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Original file line number Diff line number Diff line change
Expand Up @@ -4075,6 +4075,19 @@ def test_groupby(self):
exp = dict((key, [key]) for key in self.index)
tm.assert_dict_equal(groups, exp)

def test_index_name_retained(self):
# GH9857
result = pd.DataFrame({'x': [1, 2, 6],
'y': [2, 2, 8],
'z': [-5, 0, 5]})
result = result.set_index('z')
result.loc[10] = [9, 10]
df_expected = pd.DataFrame({'x': [1, 2, 6, 9],
'y': [2, 2, 8, 10],
'z': [-5, 0, 5, 10]})
df_expected = df_expected.set_index('z')
tm.assert_frame_equal(result, df_expected)


def test_get_combined_index():
from pandas.core.index import _get_combined_index
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