Merge pull request #10826 from sinhrks/merge_empty

BUG: Merge with empty dataframe may raise IndexError

Author: jreback

doc/source/whatsnew/v0.17.0.txt

@@ -685,6 +685,8 @@ 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`)
 

pandas/tools/merge.py

@@ -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
@@ -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
@@ -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)))
 

pandas/tools/tests/test_merge.py

@@ -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?