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BUG: Merge with empty dataframe may raise IndexError #10826

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Merged
merged 1 commit into from
Aug 20, 2015
Merged

BUG: Merge with empty dataframe may raise IndexError #10826

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2 changes: 2 additions & 0 deletions doc/source/whatsnew/v0.17.0.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -660,6 +660,8 @@ Bug Fixes
- Bug in ``io.common.get_filepath_or_buffer`` which caused reading of valid S3 files to fail if the bucket also contained keys for which the user does not have read permission (:issue:`10604`)
- Bug in vectorised setting of timestamp columns with python ``datetime.date`` and numpy ``datetime64`` (:issue:`10408`, :issue:`10412`)
- Bug in ``Index.take`` may add unnecessary ``freq`` attribute (:issue:`10791`)
- Bug in ``merge`` with empty ``DataFrame`` may raise ``IndexError`` (:issue:`10824`)


- Bug in ``read_csv`` when using the ``nrows`` or ``chunksize`` parameters if file contains only a header line (:issue:`9535`)

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18 changes: 16 additions & 2 deletions pandas/tools/merge.py
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Original file line number Diff line number Diff line change
Expand Up @@ -217,6 +217,9 @@ def _maybe_add_join_keys(self, result, left_indexer, right_indexer):
if left_indexer is not None and right_indexer is not None:

if name in self.left:
if len(self.left) == 0:
continue

na_indexer = (left_indexer == -1).nonzero()[0]
if len(na_indexer) == 0:
continue
Expand All @@ -226,6 +229,9 @@ def _maybe_add_join_keys(self, result, left_indexer, right_indexer):
na_indexer, com.take_1d(self.right_join_keys[i],
right_na_indexer))
elif name in self.right:
if len(self.right) == 0:
continue

na_indexer = (right_indexer == -1).nonzero()[0]
if len(na_indexer) == 0:
continue
Expand Down Expand Up @@ -270,9 +276,17 @@ def _get_join_info(self):
sort=self.sort, how=self.how)

if self.right_index:
join_index = self.left.index.take(left_indexer)
if len(self.left) > 0:
join_index = self.left.index.take(left_indexer)
else:
join_index = self.right.index.take(right_indexer)
left_indexer = np.array([-1] * len(join_index))
elif self.left_index:
join_index = self.right.index.take(right_indexer)
if len(self.right) > 0:
join_index = self.right.index.take(right_indexer)
else:
join_index = self.left.index.take(left_indexer)
right_indexer = np.array([-1] * len(join_index))
else:
join_index = Index(np.arange(len(left_indexer)))

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82 changes: 82 additions & 0 deletions pandas/tools/tests/test_merge.py
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Expand Up @@ -737,6 +737,88 @@ def test_left_merge_empty_dataframe(self):
result = merge(right, left, on='key', how='right')
assert_frame_equal(result, left)

def test_merge_left_empty_right_empty(self):
# GH 10824
left = pd.DataFrame([], columns=['a', 'b', 'c'])
right = pd.DataFrame([], columns=['x', 'y', 'z'])

exp_in = pd.DataFrame([], columns=['a', 'b', 'c', 'x', 'y', 'z'],
dtype=object)

for kwarg in [dict(left_index=True, right_index=True),
dict(left_index=True, right_on='x'),
dict(left_on='a', right_index=True),
dict(left_on='a', right_on='x')]:

result = pd.merge(left, right, how='inner', **kwarg)
tm.assert_frame_equal(result, exp_in)
result = pd.merge(left, right, how='left', **kwarg)
tm.assert_frame_equal(result, exp_in)
result = pd.merge(left, right, how='right', **kwarg)
tm.assert_frame_equal(result, exp_in)
result = pd.merge(left, right, how='outer', **kwarg)
tm.assert_frame_equal(result, exp_in)

def test_merge_left_empty_right_notempty(self):
# GH 10824
left = pd.DataFrame([], columns=['a', 'b', 'c'])
right = pd.DataFrame([[1, 2, 3], [4, 5, 6], [7, 8, 9]],
columns=['x', 'y', 'z'])

exp_out = pd.DataFrame({'a': np.array([np.nan]*3, dtype=object),
'b': np.array([np.nan]*3, dtype=object),
'c': np.array([np.nan]*3, dtype=object),
'x': [1, 4, 7],
'y': [2, 5, 8],
'z': [3, 6, 9]},
columns=['a', 'b', 'c', 'x', 'y', 'z'])
exp_in = exp_out[0:0] # make empty DataFrame keeping dtype

for kwarg in [dict(left_index=True, right_index=True),
dict(left_index=True, right_on='x'),
dict(left_on='a', right_index=True),
dict(left_on='a', right_on='x')]:

result = pd.merge(left, right, how='inner', **kwarg)
tm.assert_frame_equal(result, exp_in)
result = pd.merge(left, right, how='left', **kwarg)
tm.assert_frame_equal(result, exp_in)

result = pd.merge(left, right, how='right', **kwarg)
tm.assert_frame_equal(result, exp_out)
result = pd.merge(left, right, how='outer', **kwarg)
tm.assert_frame_equal(result, exp_out)

def test_merge_left_notempty_right_empty(self):
# GH 10824
left = pd.DataFrame([[1, 2, 3], [4, 5, 6], [7, 8, 9]],
columns=['a', 'b', 'c'])
right = pd.DataFrame([], columns=['x', 'y', 'z'])

exp_out = pd.DataFrame({'a': [1, 4, 7],
'b': [2, 5, 8],
'c': [3, 6, 9],
'x': np.array([np.nan]*3, dtype=object),
'y': np.array([np.nan]*3, dtype=object),
'z': np.array([np.nan]*3, dtype=object)},
columns=['a', 'b', 'c', 'x', 'y', 'z'])
exp_in = exp_out[0:0] # make empty DataFrame keeping dtype

for kwarg in [dict(left_index=True, right_index=True),
dict(left_index=True, right_on='x'),
dict(left_on='a', right_index=True),
dict(left_on='a', right_on='x')]:

result = pd.merge(left, right, how='inner', **kwarg)
tm.assert_frame_equal(result, exp_in)
result = pd.merge(left, right, how='right', **kwarg)
tm.assert_frame_equal(result, exp_in)

result = pd.merge(left, right, how='left', **kwarg)
tm.assert_frame_equal(result, exp_out)
result = pd.merge(left, right, how='outer', **kwarg)
tm.assert_frame_equal(result, exp_out)

def test_merge_nosort(self):
# #2098, anything to do?

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