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ENH: allow duplicate column names if they are not merged upon #10639

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ENH: allow duplicate column names if they are not merged upon #10639

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0d49d8a
allow duplicate column names if they are not merged upon
jdrudolph Jul 20, 2015
05ba2df
release note
jdrudolph Jul 23, 2015
4e850d4
clarified whatsnew
jdrudolph Jul 27, 2015
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2 changes: 2 additions & 0 deletions doc/source/whatsnew/v0.17.0.txt
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Expand Up @@ -80,6 +80,8 @@ Other enhancements

- ``regex`` argument to ``DataFrame.filter`` now handles numeric column names instead of raising ``ValueError`` (:issue:`10384`).

- ``pd.merge`` will now allow duplicate column names if they are not merged upon (:issue:`10639`).

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use pd.merge

if you were a user and saw this, would you want to see a mini-example? (and should we add a mini-example to the merge docs proper?)

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i don't think there is a need for an example since the behavior now is very natural. Merging 'just works' despite some duplicate column names which are irrelevant to the merge process.

.. _whatsnew_0170.api:

.. _whatsnew_0170.api_breaking:
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11 changes: 3 additions & 8 deletions pandas/tools/merge.py
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Expand Up @@ -402,19 +402,14 @@ def _validate_specification(self):
if self.left_on is None:
raise MergeError('Must pass left_on or left_index=True')
else:
if not self.left.columns.is_unique:
raise MergeError("Left data columns not unique: %s"
% repr(self.left.columns))

if not self.right.columns.is_unique:
raise MergeError("Right data columns not unique: %s"
% repr(self.right.columns))

# use the common columns
common_cols = self.left.columns.intersection(
self.right.columns)
if len(common_cols) == 0:
raise MergeError('No common columns to perform merge on')
if not common_cols.is_unique:
raise MergeError("Data columns not unique: %s"
% repr(common_cols))
self.left_on = self.right_on = common_cols
elif self.on is not None:
if self.left_on is not None or self.right_on is not None:
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10 changes: 9 additions & 1 deletion pandas/tools/tests/test_merge.py
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Expand Up @@ -843,7 +843,6 @@ def test_join_append_timedeltas(self):
assert_frame_equal(result, expected)

def test_overlapping_columns_error_message(self):
# #2649
df = DataFrame({'key': [1, 2, 3],
'v1': [4, 5, 6],
'v2': [7, 8, 9]})
Expand All @@ -853,7 +852,16 @@ def test_overlapping_columns_error_message(self):

df.columns = ['key', 'foo', 'foo']
df2.columns = ['key', 'bar', 'bar']
expected = DataFrame({'key': [1, 2, 3],
'v1': [4, 5, 6],
'v2': [7, 8, 9],
'v3': [4, 5, 6],
'v4': [7, 8, 9]})
expected.columns = ['key', 'foo', 'foo', 'bar', 'bar']
assert_frame_equal(merge(df, df2), expected)

# #2649
df2.columns = ['key1', 'foo', 'foo']
self.assertRaises(ValueError, merge, df, df2)

def _check_merge(x, y):
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