prevent setting fields on custom config

pydantic/decorator.py

@@ -44,7 +44,7 @@ def wrapper_function(*args: Any, **kwargs: Any) -> Any:
 
 
 class ValidatedFunction:
-    def __init__(self, function: 'Callable', config: 'ConfigType'):
+    def __init__(self, function: 'Callable', config: 'ConfigType'):  # noqa C901
         from inspect import Parameter, signature
 
         parameters: Mapping[str, Parameter] = signature(function).parameters
@@ -55,6 +55,21 @@ def __init__(self, function: 'Callable', config: 'ConfigType'):
                 f'names when using the "{validate_arguments.__name__}" decorator'
             )
 
+        class CustomConfig:
+            pass
+
+        if not TYPE_CHECKING:  # pragma: no branch
+            if isinstance(config, dict):
+                CustomConfig = type('Config', (), config)  # noqa: F811
+            elif config is not None:
+                CustomConfig = config  # noqa: F811
+
+        if hasattr(CustomConfig, 'fields'):
+            raise ConfigError(
+                'Setting the "fields" property on custom Config for '
+                '@validate_arguments is not yet supported, please remove.'
+            )
+
         self.raw_function = function
         self.arg_mapping: Dict[int, str] = {}
         self.positional_only_args = set()
@@ -108,7 +123,7 @@ def __init__(self, function: 'Callable', config: 'ConfigType'):
             # same with kwargs
             fields[self.v_kwargs_name] = Dict[Any, Any], None
 
-        self.create_model(fields, takes_args, takes_kwargs, config)
+        self.create_model(fields, takes_args, takes_kwargs, CustomConfig)
 
     def call(self, *args: Any, **kwargs: Any) -> Any:
         values = self.build_values(args, kwargs)
@@ -178,18 +193,11 @@ def execute(self, m: BaseModel) -> Any:
         else:
             return self.raw_function(**d)
 
-    def create_model(self, fields: Dict[str, Any], takes_args: bool, takes_kwargs: bool, config: 'ConfigType') -> None:
+    def create_model(
+        self, fields: Dict[str, Any], takes_args: bool, takes_kwargs: bool, CustomConfig: Type[Any]
+    ) -> None:
         pos_args = len(self.arg_mapping)
 
-        class CustomConfig:
-            pass
-
-        if not TYPE_CHECKING:
-            if isinstance(config, dict):
-                CustomConfig = type('Config', (), config)  # noqa: F811
-            elif config is not None:
-                CustomConfig = config  # noqa: F811
-
         class DecoratorBaseModel(BaseModel):
             @validator(self.v_args_name, check_fields=False, allow_reuse=True)
             def check_args(cls, v: List[Any]) -> List[Any]:

setup.cfg

@@ -1,6 +1,5 @@
 [tool:pytest]
 testpaths = tests
-timeout = 10
 filterwarnings =
     error
     ignore::DeprecationWarning:distutils

tests/test_decorator.py

@@ -265,15 +265,55 @@ def foo(cls, a: int, b: int):
     ]
 
 
-def test_config():
-    @validate_arguments(config=dict(title='testing', fields={'b': 'bang'}))
+def test_config_title():
+    @validate_arguments(config=dict(title='Testing'))
     def foo(a: int, b: int):
         return f'{a}, {b}'
 
     assert foo(1, 2) == '1, 2'
-    assert foo(1, bang=2) == '1, 2'
-    assert foo.model.schema()['title'] == 'testing'
+    assert foo(1, b=2) == '1, 2'
+    assert foo.model.schema()['title'] == 'Testing'
+
+
+def test_config_title_cls():
+    class Config:
+        title = 'Testing'
+
+    @validate_arguments(config=Config)
+    def foo(a: int, b: int):
+        return f'{a}, {b}'
+
+    assert foo(1, 2) == '1, 2'
+    assert foo(1, b=2) == '1, 2'
+    assert foo.model.schema()['title'] == 'Testing'
+
+
+def test_config_fields():
+    with pytest.raises(ConfigError, match='Setting the "fields" property on custom Config for'):
+
+        @validate_arguments(config=dict(fields={'b': 'bang'}))
+        def foo(a: int, b: int):
+            return f'{a}, {b}'
+
+
+def test_config_arbitrary_types_allowed():
+    class EggBox:
+        def __str__(self) -> str:
+            return 'EggBox()'
+
+    @validate_arguments(config=dict(arbitrary_types_allowed=True))
+    def foo(a: int, b: EggBox):
+        return f'{a}, {b}'
+
+    assert foo(1, EggBox()) == '1, EggBox()'
     with pytest.raises(ValidationError) as exc_info:
-        foo(1, b=2)
+        assert foo(1, 2) == '1, 2'
 
-    assert exc_info.value.errors() == ...
+    assert exc_info.value.errors() == [
+        {
+            'loc': ('b',),
+            'msg': 'instance of EggBox expected',
+            'type': 'type_error.arbitrary_type',
+            'ctx': {'expected_arbitrary_type': 'EggBox'},
+        },
+    ]