Add Config.keep_untouched for custom descriptors support (#679)

* Add Config.keep_untouched for custom descriptors support

* Update HISTORY.rst

* Separate test

* Fix tests

* Update pydantic/validators.py

Co-Authored-By: Samuel Colvin <samcolvin@gmail.com>

* Update HISTORY.rst

Co-Authored-By: Samuel Colvin <samcolvin@gmail.com>

* Rename TYPE_BLACKLIST -> UNTOUCHED_TYPES, fix tests and formatting

* Update docs/index.rst

Co-Authored-By: Samuel Colvin <samcolvin@gmail.com>

HISTORY.rst

@@ -12,6 +12,7 @@ v0.31 (unreleased)
 * add advanced exclude support for ``dict``, ``json`` and ``copy``, #648 by @MrMrRobat
 * fix bug in ``GenericModel`` for models with concrete parameterized fields, #672 by @dmontagu
 * add documentation for Literal type, #651 by @dmontagu
+* add ``Config.keep_untouched`` for custom descriptors support, #679 by @MrMrRobat
 * use ``inspect.cleandoc`` internally to get model description, #657 by @tiangolo
 * add Color to schema generation, by @euri10
 

docs/index.rst

@@ -762,6 +762,8 @@ Options:
     details.
 :orm_mode: allows usage of :ref:`ORM mode <orm_mode>`
 :alias_generator: callable that takes field name and returns alias for it
+:keep_untouched: tuple of types (e. g. descriptors) that won't change during model creation and won't be
+  included in the model schemas.
 
 .. warning::
 

pydantic/main.py

@@ -97,6 +97,7 @@ class BaseConfig:
     json_encoders: Dict[AnyType, AnyCallable] = {}
     orm_mode: bool = False
     alias_generator: Optional[Callable[[str], str]] = None
+    keep_untouched: Tuple[type, ...] = ()
 
     @classmethod
     def get_field_schema(cls, name: str) -> Dict[str, str]:
@@ -169,7 +170,7 @@ def validate_custom_root_type(fields: Dict[str, Field]) -> None:
         raise TypeError('custom root type cannot allow mapping')
 
 
-TYPE_BLACKLIST = FunctionType, property, type, classmethod, staticmethod
+UNTOUCHED_TYPES = FunctionType, property, type, classmethod, staticmethod
 
 
 class MetaModel(ABCMeta):
@@ -217,10 +218,11 @@ class MetaModel(ABCMeta):
                         config=config,
                     )
 
+            untouched_types = UNTOUCHED_TYPES + config.keep_untouched
             for var_name, value in namespace.items():
                 if (
                     is_valid_field(var_name)
-                    and (annotations.get(var_name) == PyObject or not isinstance(value, TYPE_BLACKLIST))
+                    and (annotations.get(var_name) == PyObject or not isinstance(value, untouched_types))
                     and var_name not in class_vars
                 ):
                     validate_field_name(bases, var_name)

pydantic/validators.py

@@ -475,7 +475,9 @@ def find_validators(  # noqa: C901 (ignore complexity)
     if config.arbitrary_types_allowed:
         yield make_arbitrary_type_validator(type_)
     else:
-        raise RuntimeError(f'no validator found for {type_}')
+        raise RuntimeError(
+            f'no validator found for {type_} see `keep_untouched` or `arbitrary_types_allowed` in Config'
+        )
 
 
 def _find_supertype(type_: AnyType) -> Optional[AnyType]:

tests/test_main.py

@@ -743,3 +743,66 @@ def test_parse_root_as_mapping():
 
         class MyModel(BaseModel):
             __root__: Mapping[str, str]
+
+
+def test_untouched_types():
+    from pydantic import BaseModel
+
+    class _ClassPropertyDescriptor:
+        def __init__(self, getter):
+            self.getter = getter
+
+        def __get__(self, instance, owner):
+            return self.getter(owner)
+
+    classproperty = _ClassPropertyDescriptor
+
+    class Model(BaseModel):
+        class Config:
+            keep_untouched = (classproperty,)
+
+        @classproperty
+        def class_name(cls) -> str:
+            return cls.__name__
+
+    assert Model.class_name == 'Model'
+    assert Model().class_name == 'Model'
+
+
+def test_custom_types_fail_without_keep_untouched():
+    from pydantic import BaseModel
+
+    class _ClassPropertyDescriptor:
+        def __init__(self, getter):
+            self.getter = getter
+
+        def __get__(self, instance, owner):
+            return self.getter(owner)
+
+    classproperty = _ClassPropertyDescriptor
+
+    with pytest.raises(RuntimeError) as e:
+
+        class Model(BaseModel):
+            @classproperty
+            def class_name(cls) -> str:
+                return cls.__name__
+
+        Model.class_name
+
+    assert str(e.value) == (
+        "no validator found for <class 'tests.test_main.test_custom_types_fail_without_keep_untouched.<locals>."
+        "_ClassPropertyDescriptor'> see `keep_untouched` or `arbitrary_types_allowed` in Config"
+    )
+
+    class Model(BaseModel):
+        class Config:
+            arbitrary_types_allowed = True
+
+        @classproperty
+        def class_name(cls) -> str:
+            return cls.__name__
+
+    with pytest.raises(AttributeError) as e:
+        Model.class_name
+    assert str(e.value) == "type object 'Model' has no attribute 'class_name'"