ENH: Add merge type validation on pandas.merge

doc/source/whatsnew/v3.0.0.rst

@@ -30,6 +30,7 @@ Other enhancements
 ^^^^^^^^^^^^^^^^^^
 - :class:`pandas.api.typing.FrozenList` is available for typing the outputs of :attr:`MultiIndex.names`, :attr:`MultiIndex.codes` and :attr:`MultiIndex.levels` (:issue:`58237`)
 - :class:`pandas.api.typing.SASReader` is available for typing the output of :func:`read_sas` (:issue:`55689`)
+- :func:`pandas.merge` now validates the ``how`` parameter input (merge type) (:issue:`59435`)
 - :func:`DataFrame.to_excel` now raises an ``UserWarning`` when the character count in a cell exceeds Excel's limitation of 32767 characters (:issue:`56954`)
 - :func:`read_stata` now returns ``datetime64`` resolutions better matching those natively stored in the stata format (:issue:`55642`)
 - :meth:`DataFrame.agg` called with ``axis=1`` and a ``func`` which relabels the result index now raises a ``NotImplementedError`` (:issue:`58807`).

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,