Fix error message (#2246)

close #2245

CHANGELOG.rst

@@ -1,6 +1,14 @@
 Changelog
 ---------
 
+3.21.1 (unreleased)
+*******************
+
+Bug fixes:
+
+- Fix error message when field is declared as a class and not an instance (:issue:`2245`).
+  Thanks :user:`travnick` for reporting.
+
 3.21.0 (2024-02-26)
 *******************
 

src/marshmallow/schema.py

@@ -1047,9 +1047,9 @@ def _bind_field(self, field_name: str, field_obj: ma_fields.Field) -> None:
             # the type checker's perspective.
             if isinstance(field_obj, type) and issubclass(field_obj, base.FieldABC):
                 msg = (
-                    'Field for "{field_name}" must be declared as a '
+                    f'Field for "{field_name}" must be declared as a '
                     "Field instance, not a class. "
-                    'Did you mean "fields.{field_obj.__name__}()"?'
+                    f'Did you mean "fields.{field_obj.__name__}()"?'  # type: ignore
                 )
                 raise TypeError(msg) from error
             raise error

tests/test_schema.py

@@ -553,7 +553,9 @@ def test_fields_must_be_declared_as_instances(user):
     class BadUserSchema(Schema):
         name = fields.String
 
-    with pytest.raises(TypeError, match="must be declared as a Field instance"):
+    with pytest.raises(
+        TypeError, match='Field for "name" must be declared as a Field instance'
+    ):
         BadUserSchema().dump(user)