ENH Better error message needed when accidentall… (scikit-learn#16861)

Author: MichaelShoemaker

sklearn/model_selection/_search.py

@@ -385,9 +385,10 @@ def _check_param_grid(param_grid):
 
             if (isinstance(v, str) or
                     not isinstance(v, (np.ndarray, Sequence))):
-                raise ValueError("Parameter values for parameter ({0}) need "
-                                 "to be a sequence(but not a string) or"
-                                 " np.ndarray.".format(name))
+                raise ValueError("Parameter grid for parameter ({0}) needs to"
+                                 " be a list or numpy array, but got ({1})."
+                                 " Single values need to be wrapped in a list"
+                                 " with one element.".format(name, type(v)))
 
             if len(v) == 0:
                 raise ValueError("Parameter values for parameter ({0}) need "

sklearn/model_selection/tests/test_search.py

@@ -428,12 +428,14 @@ def test_grid_search_when_param_grid_includes_range():
 
 
 def test_grid_search_bad_param_grid():
-    param_dict = {"C": 1.0}
+    param_dict = {"C": 1}
     clf = SVC(gamma='auto')
     assert_raise_message(
         ValueError,
-        "Parameter values for parameter (C) need to be a sequence"
-        "(but not a string) or np.ndarray.",
+        "Parameter grid for parameter (C) needs to"
+        " be a list or numpy array, but got (<class 'int'>)."
+        " Single values need to be wrapped in a list"
+        " with one element.",
         GridSearchCV, clf, param_dict)
 
     param_dict = {"C": []}
@@ -447,8 +449,10 @@ def test_grid_search_bad_param_grid():
     clf = SVC(gamma='auto')
     assert_raise_message(
         ValueError,
-        "Parameter values for parameter (C) need to be a sequence"
-        "(but not a string) or np.ndarray.",
+        "Parameter grid for parameter (C) needs to"
+        " be a list or numpy array, but got (<class 'str'>)."
+        " Single values need to be wrapped in a list"
+        " with one element.",
         GridSearchCV, clf, param_dict)
 
     param_dict = {"C": np.ones((3, 2))}