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Backport PR pandas-dev#24916: BUG-24212 fix regression in pandas-dev#…

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meeseeksmachine authored and jreback committed Jan 26, 2019
1 parent 31dcbb7 commit 84056c52e3f20ab44921b86a8e0f05275bf8ddb4
Showing with 60 additions and 19 deletions.
  1. +3 −0 doc/source/whatsnew/v0.24.1.rst
  2. +43 −2 pandas/core/reshape/merge.py
  3. +14 −17 pandas/tests/reshape/merge/test_merge.py
@@ -63,6 +63,9 @@ Bug Fixes
-
-

**Reshaping**

- Bug in :func:`merge` when merging by index name would sometimes result in an incorrectly numbered index (:issue:`24212`)

**Other**

@@ -757,13 +757,21 @@ def _get_join_info(self):

if self.right_index:
if len(self.left) > 0:
join_index = self.left.index.take(left_indexer)
join_index = self._create_join_index(self.left.index,
self.right.index,
left_indexer,
right_indexer,
how='right')
else:
join_index = self.right.index.take(right_indexer)
left_indexer = np.array([-1] * len(join_index))
elif self.left_index:
if len(self.right) > 0:
join_index = self.right.index.take(right_indexer)
join_index = self._create_join_index(self.right.index,
self.left.index,
right_indexer,
left_indexer,
how='left')
else:
join_index = self.left.index.take(left_indexer)
right_indexer = np.array([-1] * len(join_index))
@@ -774,6 +782,39 @@ def _get_join_info(self):
join_index = join_index.astype(object)
return join_index, left_indexer, right_indexer

def _create_join_index(self, index, other_index, indexer,
other_indexer, how='left'):
"""
Create a join index by rearranging one index to match another
Parameters
----------
index: Index being rearranged
other_index: Index used to supply values not found in index
indexer: how to rearrange index
how: replacement is only necessary if indexer based on other_index
Returns
-------
join_index
"""
join_index = index.take(indexer)
if (self.how in (how, 'outer') and
not isinstance(other_index, MultiIndex)):
# if final index requires values in other_index but not target
# index, indexer may hold missing (-1) values, causing Index.take
# to take the final value in target index
mask = indexer == -1
if np.any(mask):
# if values missing (-1) from target index,
# take from other_index instead
join_list = join_index.to_numpy()
other_list = other_index.take(other_indexer).to_numpy()
join_list[mask] = other_list[mask]
join_index = Index(join_list, dtype=join_index.dtype,
name=join_index.name)
return join_index

def _get_merge_keys(self):
"""
Note: has side effects (copy/delete key columns)
@@ -939,25 +939,22 @@ def test_merge_two_empty_df_no_division_error(self):
with np.errstate(divide='raise'):
merge(a, a, on=('a', 'b'))

@pytest.mark.parametrize('how', ['left', 'outer'])
@pytest.mark.xfail(reason="GH-24897")
@pytest.mark.parametrize('how', ['right', 'outer'])
def test_merge_on_index_with_more_values(self, how):
# GH 24212
# pd.merge gets [-1, -1, 0, 1] as right_indexer, ensure that -1 is
# interpreted as a missing value instead of the last element
df1 = pd.DataFrame([[1, 2], [2, 4], [3, 6], [4, 8]],
columns=['a', 'b'])
df2 = pd.DataFrame([[3, 30], [4, 40]],
columns=['a', 'c'])
df1.set_index('a', drop=False, inplace=True)
df2.set_index('a', inplace=True)
result = pd.merge(df1, df2, left_index=True, right_on='a', how=how)
expected = pd.DataFrame([[1, 2, np.nan],
[2, 4, np.nan],
[3, 6, 30.0],
[4, 8, 40.0]],
columns=['a', 'b', 'c'])
expected.set_index('a', drop=False, inplace=True)
# pd.merge gets [0, 1, 2, -1, -1, -1] as left_indexer, ensure that
# -1 is interpreted as a missing value instead of the last element
df1 = pd.DataFrame({'a': [1, 2, 3], 'key': [0, 2, 2]})
df2 = pd.DataFrame({'b': [1, 2, 3, 4, 5]})
result = df1.merge(df2, left_on='key', right_index=True, how=how)
expected = pd.DataFrame([[1.0, 0, 1],
[2.0, 2, 3],
[3.0, 2, 3],
[np.nan, 1, 2],
[np.nan, 3, 4],
[np.nan, 4, 5]],
columns=['a', 'key', 'b'])
expected.set_index(Int64Index([0, 1, 2, 1, 3, 4]), inplace=True)
assert_frame_equal(result, expected)

def test_merge_right_index_right(self):

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