BUG:Sanity check on merge parameters for correct exception #26824

pandas/core/reshape/merge.py

@@ -1089,13 +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
+ if not self.right_on:
+ 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
+ if not self.left_on:
+ 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

@@ -1763,3 +1763,23 @@ 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_merge_correct_exception(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 = 'both left_on and right_on should be passed'
+ if merge_type == 'left_on':
+ with pytest.raises(ValueError, match=msg):
+ pd.merge(df1, df2, how='left', left_on='A')
+ if merge_type == 'right_on':
+ with pytest.raises(ValueError, match=msg):
+ pd.merge(df1, df2, how='left', right_on='A')