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BUG: preserve left frame order in left merge #8948

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Merged
merged 1 commit into from
Dec 1, 2014
Merged

BUG: preserve left frame order in left merge #8948

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1 change: 1 addition & 0 deletions doc/source/whatsnew/v0.15.2.txt
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Expand Up @@ -101,6 +101,7 @@ Bug Fixes
- ``slice`` string method now takes step into account (:issue:`8754`)
- Bug in ``BlockManager`` where setting values with different type would break block integrity (:issue:`8850`)
- Bug in ``DatetimeIndex`` when using ``time`` object as key (:issue:`8667`)
- Bug in ``merge`` where ``how='left'`` and ``sort=False`` would not preserve left frame order (:issue:`7331`)
- Fix negative step support for label-based slices (:issue:`8753`)

Old behavior:
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4 changes: 3 additions & 1 deletion pandas/tools/merge.py
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Expand Up @@ -479,8 +479,10 @@ def _get_join_indexers(left_keys, right_keys, sort=False, how='inner'):
left_group_key, right_group_key, max_groups = \
_factorize_keys(left_group_key, right_group_key, sort=sort)

# preserve left frame order if how == 'left' and sort == False
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is this the only case where it needs/should be preserved? (e.g. what about other how's)

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in pandas/src/join.pyx only left_outer_join accepts this extra argument sort, which was added in this commit to preserve left frame order.

I think it makes sense, because except for the case of left join preserving order is not well defined or I do not see an immediate use for it.

kwargs = {'sort':sort} if how == 'left' else {}
join_func = _join_functions[how]
return join_func(left_group_key, right_group_key, max_groups)
return join_func(left_group_key, right_group_key, max_groups, **kwargs)


class _OrderedMerge(_MergeOperation):
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12 changes: 12 additions & 0 deletions pandas/tools/tests/test_merge.py
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Expand Up @@ -1022,6 +1022,13 @@ def test_left_join_index_multi_match_multiindex(self):

tm.assert_frame_equal(result, expected)

# GH7331 - maintain left frame order in left merge
right.reset_index(inplace=True)
right.columns = left.columns[:3].tolist() + right.columns[-1:].tolist()
result = merge(left, right, how='left', on=left.columns[:-1].tolist())
expected.index = np.arange(len(expected))
tm.assert_frame_equal(result, expected)

def test_left_join_index_multi_match(self):
left = DataFrame([
['c', 0],
Expand Down Expand Up @@ -1059,6 +1066,11 @@ def test_left_join_index_multi_match(self):

tm.assert_frame_equal(result, expected)

# GH7331 - maintain left frame order in left merge
result = merge(left, right.reset_index(), how='left', on='tag')
expected.index = np.arange(len(expected))
tm.assert_frame_equal(result, expected)

def test_join_multi_dtypes(self):

# test with multi dtypes in the join index
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