Fixes #55884- Added else for invalid fill_method (#55927)

* Fixes #55884- Added else for invalid fill_method

* Fixes #55884- Added else for invalid fill_method

* Update pandas/tests/reshape/merge/test_merge_ordered.py

Co-authored-by: Matthew Roeschke <10647082+mroeschke@users.noreply.github.com>

* Update pandas/tests/reshape/merge/test_merge_ordered.py

Co-authored-by: Matthew Roeschke <10647082+mroeschke@users.noreply.github.com>

* Update pandas/core/reshape/merge.py

Co-authored-by: Matthew Roeschke <10647082+mroeschke@users.noreply.github.com>

* Fixed mergeordered ValueError Message

* Use pytest fixtures & parametrize for tests

---------

Co-authored-by: Matthew Roeschke <10647082+mroeschke@users.noreply.github.com>

Author: ziad-kermadi

pandas/core/reshape/merge.py

@@ -1863,9 +1863,11 @@ def get_result(self, copy: bool | None = True) -> DataFrame:
             right_indexer = cast("npt.NDArray[np.intp]", right_indexer)
             left_join_indexer = libjoin.ffill_indexer(left_indexer)
             right_join_indexer = libjoin.ffill_indexer(right_indexer)
-        else:
+        elif self.fill_method is None:
             left_join_indexer = left_indexer
             right_join_indexer = right_indexer
+        else:
+            raise ValueError("fill_method must be 'ffill' or None")
 
         result = self._reindex_and_concat(
             join_index, left_join_indexer, right_join_indexer, copy=copy

pandas/tests/reshape/merge/test_merge_ordered.py

@@ -1,3 +1,5 @@
+import re
+
 import numpy as np
 import pytest
 
@@ -209,3 +211,11 @@ def test_elements_not_in_by_but_in_df(self):
         msg = r"\{'h'\} not found in left columns"
         with pytest.raises(KeyError, match=msg):
             merge_ordered(left, right, on="E", left_by=["G", "h"])
+
+    @pytest.mark.parametrize("invalid_method", ["linear", "carrot"])
+    def test_ffill_validate_fill_method(self, left, right, invalid_method):
+        # GH 55884
+        with pytest.raises(
+            ValueError, match=re.escape("fill_method must be 'ffill' or None")
+        ):
+            merge_ordered(left, right, on="key", fill_method=invalid_method)