Move dtype check to _get_merge_keys

pandas/core/reshape/merge.py

@@ -1637,6 +1637,7 @@ def _get_merge_keys(self):
 
  # note this function has side effects
  (left_join_keys, right_join_keys, join_names) = super()._get_merge_keys()
+ len_by_cols = 0 if self.left_by is None else len(self.left_by)
 
  # validate index types are the same
  for i, (lk, rk) in enumerate(zip(left_join_keys, right_join_keys)):
@@ -1662,6 +1663,13 @@ def _get_merge_keys(self):
  )
  raise MergeError(msg)
 
+ if i >= len_by_cols:
+ if is_object_dtype(rk.dtype) or is_object_dtype(lk.dtype):
+ raise ValueError(
+ f"Incompatible merge [{i}] dtype, {repr(lk.dtype)} and "
+ f"{repr(rk.dtype)}, both sides must have numeric dtype"
+ )
+
  # validate tolerance; datetime.timedelta or Timedelta if we have a DTI
  if self.tolerance is not None:
 
@@ -1731,12 +1739,6 @@ def flip(xs) -> np.ndarray:
  )
  tolerance = self.tolerance
 
- if is_object_dtype(right_values.dtype) or is_object_dtype(left_values.dtype):
- raise ValueError(
- f"Incompatible merge dtype, {repr(left_values.dtype)} and "
- f"{repr(right_values.dtype)}, both sides must have numeric dtype"
- )
-
  # we require sortedness and non-null values in the join keys
  if not Index(left_values).is_monotonic:
  side = "left"

pandas/tests/reshape/merge/test_merge_asof.py

@@ -1351,6 +1351,16 @@ def test_non_numerical_dtype(data):
  right = pd.DataFrame({"x": data})
  with pytest.raises(
  ValueError,
- match="Incompatible merge dtype, .*, both sides must have numeric dtype",
+ match=r"Incompatible merge \[0\] dtype, .*, both sides must have numeric dtype",
  ):
  pd.merge_asof(left, right, on="x")
+
+
+def test_some_issues():
+ left = pd.DataFrame({"a": ["12", "13", "15"], "left_val1": ["a", "b", "c"]})
+ right = pd.DataFrame({"a": ["a", "b", "c"], "left_val": ["d", "e", "f"]})
+ with pytest.raises(
+ ValueError,
+ match=r"Incompatible merge \[1\] dtype, .*, both sides must have numeric dtype",
+ ):
+ pd.merge_asof(left, right, left_on="left_val1", right_on="a", left_by="a", right_by="left_val")