Trying original patch

Author: harshit-py

pandas/core/reshape/merge.py

@@ -1089,15 +1089,19 @@ def _validate_specification(self):
                     raise ValueError('len(left_on) must equal the number '
                                      'of levels in the index of "right"')
                 self.right_on = [None] * n
-            self.right_on = self.right_on or [None] * (n + 1)
+            if self.right_on is None:
+                raise ValueError('both "left_on" and "right_on" '
+                                 'should be passed')
         elif self.right_on is not None:
             n = len(self.right_on)
             if self.left_index:
                 if len(self.right_on) != self.left.index.nlevels:
                     raise ValueError('len(right_on) must equal the number '
                                      'of levels in the index of "left"')
                 self.left_on = [None] * n
-            self.left_on = self.left_on or [None] * (n + 1)
+            if self.left_on is None:
+                raise ValueError('both "left_on" and "right_on" '
+                                 'should be passed')
         if len(self.right_on) != len(self.left_on):
             raise ValueError("len(right_on) must equal len(left_on)")
 

pandas/tests/reshape/merge/test_merge.py

@@ -1026,6 +1026,22 @@ def test_validation(self):
         result = merge(left, right, on=['a', 'b'], validate='1:1')
         assert_frame_equal(result, expected_multi)
 
+    @pytest.mark.parametrize('merge_type', ['left_on', 'right_on'])
+    def test_missing_on_raises(merge_type):
+        # GH26824
+        left = DataFrame({
+            'A': [1, 2, 3, 4, 5, 6],
+            'B': ['P', 'Q', 'R', 'S', 'T', 'U']
+        })
+        right = DataFrame({
+            'A': [1, 2, 4, 5, 7, 8],
+            'C': ['L', 'M', 'N', 'O', 'P', 'Q']
+        })
+        msg = 'should be passed'
+        kwargs = {merge_type: 'A'}
+        with pytest.raises(ValueError, match=msg):
+            pd.merge(left, right, how='left', **kwargs)
+
     def test_merge_two_empty_df_no_division_error(self):
         # GH17776, PR #17846
         a = pd.DataFrame({'a': [], 'b': [], 'c': []})
@@ -1764,19 +1780,3 @@ def test_merge_equal_cat_dtypes2():
     # Categorical is unordered, so don't check ordering.
     tm.assert_frame_equal(result, expected, check_categorical=False)
 
-
-@pytest.mark.parametrize('merge_type', ['left_on', 'right_on'])
-def test_missing_on_raises(merge_type):
-    # GH26824
-    df1 = DataFrame({
-        'A': [1, 2, 3, 4, 5, 6],
-        'B': ['P', 'Q', 'R', 'S', 'T', 'U']
-    })
-    df2 = DataFrame({
-        'A': [1, 2, 4, 5, 7, 8],
-        'C': ['L', 'M', 'N', 'O', 'P', 'Q']
-    })
-    msg = 'must equal'
-    kwargs = {merge_type: 'A'}
-    with pytest.raises(ValueError, match=msg):
-        pd.merge(df1, df2, how='left', **kwargs)