add merge type validation on pandas.merge

pandas/core/reshape/merge.py

@@ -350,7 +350,10 @@ def merge(
  left_df = _validate_operand(left)
  left._check_copy_deprecation(copy)
  right_df = _validate_operand(right)
- if how == "cross":
+ merge_type = ["left", "right", "inner", "outer", "cross"]
+ if how not in merge_type:
+ raise ValueError(f"'{how}' is not a valid Merge type ({merge_type})")
+ elif how == "cross":
  return _cross_merge(
  left_df,
  right_df,