fix: avoid RecursionError when using some types like Enum or Literal with generic models (#2438)

* fix: support properly `Enum` when combined with generic models

* whitelist iterables

* update change description

* add test for Literal

Co-authored-by: Samuel Colvin <s@muelcolvin.com>

changes/2436-PrettyWood.md

@@ -0,0 +1 @@
+Avoid `RecursionError` when using some types like `Enum` or `Literal` with generic models

pydantic/generics.py

@@ -6,7 +6,6 @@ from typing import (
     ClassVar,
     Dict,
     Generic,
-    Iterable,
     Iterator,
     List,
     Mapping,
@@ -205,13 +204,16 @@ def check_parameters_count(cls: Type[GenericModel], parameters: Tuple[Any, ...])
         raise TypeError(f'Too {description} parameters for {cls.__name__}; actual {actual}, expected {expected}')
 
 
+DictValues: Type[Any] = {}.values().__class__
+
+
 def iter_contained_typevars(v: Any) -> Iterator[TypeVarType]:
     """Recursively iterate through all subtypes and type args of `v` and yield any typevars that are found."""
     if isinstance(v, TypeVar):
         yield v
     elif hasattr(v, '__parameters__') and not get_origin(v) and lenient_issubclass(v, GenericModel):
         yield from v.__parameters__
-    elif isinstance(v, Iterable):
+    elif isinstance(v, (DictValues, list)):
         for var in v:
             yield from iter_contained_typevars(var)
     else:

tests/test_generics.py

@@ -3,6 +3,7 @@ from enum import Enum
 from typing import Any, Callable, ClassVar, Dict, Generic, List, Optional, Sequence, Tuple, Type, TypeVar, Union
 
 import pytest
+from typing_extensions import Literal
 
 from pydantic import BaseModel, Field, ValidationError, root_validator, validator
 from pydantic.generics import GenericModel, _generic_types_cache, iter_contained_typevars, replace_types
@@ -1039,3 +1040,34 @@ def test_generic_recursive_models(create_module):
     Model2 = module.Model2
     result = Model1[str].parse_obj(dict(ref=dict(ref=dict(ref=dict(ref=123)))))
     assert result == Model1(ref=Model2(ref=Model1(ref=Model2(ref='123'))))
+
+
+@skip_36
+def test_generic_enum():
+    T = TypeVar('T')
+
+    class SomeGenericModel(GenericModel, Generic[T]):
+        some_field: T
+
+    class SomeStringEnum(str, Enum):
+        A = 'A'
+        B = 'B'
+
+    class MyModel(BaseModel):
+        my_gen: SomeGenericModel[SomeStringEnum]
+
+    m = MyModel.parse_obj({'my_gen': {'some_field': 'A'}})
+    assert m.my_gen.some_field is SomeStringEnum.A
+
+
+@skip_36
+def test_generic_literal():
+    FieldType = TypeVar('FieldType')
+    ValueType = TypeVar('ValueType')
+
+    class GModel(GenericModel, Generic[FieldType, ValueType]):
+        field: Dict[FieldType, ValueType]
+
+    Fields = Literal['foo', 'bar']
+    m = GModel[Fields, str](field={'foo': 'x'})
+    assert m.dict() == {'field': {'foo': 'x'}}