Trying original patch

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 not self.right_on:
+ raise ValueError('both "right_on" and "left_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 not self.left_on:
+ raise ValueError('both "right_on" and "left_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(self, 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 = 'must equal'
+ 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': []})
@@ -1763,20 +1779,3 @@ 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)