BUG: Unclear error message when merging tables and passing invalid option to validate (#49419)

* improve the error message for invalid validate arguments

* change error message in test_join

* resolve merge conflict

Co-authored-by: Marco Edward Gorelli <[email protected]>

Author: raisadz

doc/source/whatsnew/v2.0.0.rst

@@ -477,6 +477,7 @@ Reshaping
 - Bug in :meth:`DataFrame.pivot` not respecting ``None`` as column name (:issue:`48293`)
 - Bug in :func:`join` when ``left_on`` or ``right_on`` is or includes a :class:`CategoricalIndex` incorrectly raising ``AttributeError`` (:issue:`48464`)
 - Bug in :meth:`DataFrame.pivot_table` raising ``ValueError`` with parameter ``margins=True`` when result is an empty :class:`DataFrame` (:issue:`49240`)
+- Clarified error message in :func:`merge` when passing invalid ``validate`` option (:issue:`49417`)
 
 Sparse
 ^^^^^^

pandas/core/reshape/merge.py

@@ -1568,7 +1568,18 @@ def _validate(self, validate: str) -> None:
             pass
 
         else:
-            raise ValueError("Not a valid argument for validate")
+            raise ValueError(
+                f'"{validate}" is not a valid argument. '
+                "Valid arguments are:\n"
+                '- "1:1"\n'
+                '- "1:m"\n'
+                '- "m:1"\n'
+                '- "m:m"\n'
+                '- "one_to_one"\n'
+                '- "one_to_many"\n'
+                '- "many_to_one"\n'
+                '- "many_to_many"'
+            )
 
 
 def get_join_indexers(

pandas/tests/frame/methods/test_join.py

@@ -143,7 +143,18 @@ def test_suffix_on_list_join():
 def test_join_invalid_validate(left_no_dup, right_no_dup):
     # GH 46622
     # Check invalid arguments
-    msg = "Not a valid argument for validate"
+    msg = (
+        '"invalid" is not a valid argument. '
+        "Valid arguments are:\n"
+        '- "1:1"\n'
+        '- "1:m"\n'
+        '- "m:1"\n'
+        '- "m:m"\n'
+        '- "one_to_one"\n'
+        '- "one_to_many"\n'
+        '- "many_to_one"\n'
+        '- "many_to_many"'
+    )
     with pytest.raises(ValueError, match=msg):
         left_no_dup.merge(right_no_dup, on="a", validate="invalid")
 

pandas/tests/reshape/merge/test_merge.py

@@ -1253,7 +1253,18 @@ def test_validation(self):
             merge(left_w_dups, right_w_dups, on="a", validate="one_to_many")
 
         # Check invalid arguments
-        msg = "Not a valid argument for validate"
+        msg = (
+            '"jibberish" is not a valid argument. '
+            "Valid arguments are:\n"
+            '- "1:1"\n'
+            '- "1:m"\n'
+            '- "m:1"\n'
+            '- "m:m"\n'
+            '- "one_to_one"\n'
+            '- "one_to_many"\n'
+            '- "many_to_one"\n'
+            '- "many_to_many"'
+        )
         with pytest.raises(ValueError, match=msg):
             merge(left, right, on="a", validate="jibberish")