Warn and ignore __slots__ argument to create_model (#4432)

* add test for create_model with slots

* add test for create_model with slots

* add warning and change description

* fix flakey none tests on 3.8.10

* avoid flakey coverage changes

changes/4432-samuelcolvin.md

@@ -0,0 +1 @@
+Runtime warning if `__slots__` is passed to `create_model`, `__slots__` is then ignored.

pydantic/main.py

@@ -957,6 +957,7 @@ def create_model(
     __module__: str = __name__,
     __validators__: Dict[str, 'AnyClassMethod'] = None,
     __cls_kwargs__: Dict[str, Any] = None,
+    __slots__: Optional[Tuple[str, ...]] = None,
     **field_definitions: Any,
 ) -> Type['Model']:
     """
@@ -967,13 +968,17 @@ def create_model(
     :param __module__: module of the created model
     :param __validators__: a dict of method names and @validator class methods
     :param __cls_kwargs__: a dict for class creation
+    :param __slots__: Deprecated, `__slots__` should not be passed to `create_model`
     :param field_definitions: fields of the model (or extra fields if a base is supplied)
         in the format `<name>=(<type>, <default default>)` or `<name>=<default value>, e.g.
         `foobar=(str, ...)` or `foobar=123`, or, for complex use-cases, in the format
         `<name>=<Field>` or `<name>=(<type>, <FieldInfo>)`, e.g.
         `foo=Field(datetime, default_factory=datetime.utcnow, alias='bar')` or
         `foo=(str, FieldInfo(title='Foo'))`
     """
+    if __slots__ is not None:
+        # __slots__ will be ignored from here on
+        warnings.warn('__slots__ should not be passed to create_model', RuntimeWarning)
 
     if __base__ is not None:
         if __config__ is not None:

pydantic/typing.py

@@ -333,15 +333,16 @@ def is_none_type(type_: Any) -> bool:
         return type_ in NONE_TYPES
 
 elif sys.version_info[:2] == (3, 8):
-    # We can use the fast implementation for 3.8 but there is a very weird bug
-    # where it can fail for `Literal[None]`.
-    # We just need to redefine a useless `Literal[None]` inside the function body to fix this
 
     def is_none_type(type_: Any) -> bool:
-        Literal[None]  # fix edge case
         for none_type in NONE_TYPES:
             if type_ is none_type:
                 return True
+        # With python 3.8, specifically 3.8.10, Literal "is" check sare very flakey
+        # can change on very subtle changes like use of types in other modules,
+        # hopefully this check avoids that issue.
+        if is_literal_type(type_):  # pragma: no cover
+            return all_literal_values(type_) == (None,)
         return False
 
 else:

tests/test_create_model.py

@@ -1,4 +1,4 @@
-from typing import Generic, TypeVar
+from typing import Generic, Optional, Tuple, TypeVar
 
 import pytest
 
@@ -256,3 +256,11 @@ def _some_func(self):
     # with _init_private_attributes), so the descriptor protocol won't work.
     if base is object:
         assert a._some_func == 2
+
+
+def test_create_model_with_slots():
+    field_definitions = {'__slots__': (Optional[Tuple[str, ...]], None), 'foobar': (Optional[int], None)}
+    with pytest.warns(RuntimeWarning, match='__slots__ should not be passed to create_model'):
+        model = create_model('PartialPet', **field_definitions)
+
+    assert model.__fields__.keys() == {'foobar'}