Bug 18068: Enumerates on keys for checking dtypes

Author: manrajgrover

pandas/core/reshape/merge.py

@@ -1253,11 +1253,12 @@ def _get_merge_keys(self):
          join_names) = super(_AsOfMerge, self)._get_merge_keys()
 
         # validate index types are the same
-        for lk, rk in zip(left_join_keys, right_join_keys):
+        for i, (lk, rk) in enumerate(zip(left_join_keys, right_join_keys)):
             if not is_dtype_equal(lk.dtype, rk.dtype):
-                raise MergeError("incompatible merge keys {lkdtype} and "
-                                 "{rkdtype}, must be the same type"
-                                 .format(lkdtype=lk.dtype, rkdtype=rk.dtype))
+                raise MergeError("incompatible merge keys [{i} {lkdtype} and "
+                                 "{rkdtype}], must be the same type"
+                                 .format(i=i, lkdtype=lk.dtype,
+                                         rkdtype=rk.dtype))
 
         # validate tolerance; must be a Timedelta if we have a DTI
         if self.tolerance is not None: