fix: support underscore_attrs_are_private with generic models (#2139)

closes #2138

Author: PrettyWood

changes/2138-PrettyWood.md

@@ -0,0 +1 @@
+fix: support `underscore_attrs_are_private` with generic models

pydantic/utils.py

@@ -636,5 +636,6 @@ def is_valid_private_name(name: str) -> bool:
         '__classcell__',
         '__doc__',
         '__module__',
+        '__orig_bases__',
         '__qualname__',
     }

tests/test_private_attributes.py

@@ -1,9 +1,13 @@
-from typing import ClassVar
+import sys
+from typing import ClassVar, Generic, TypeVar
 
 import pytest
 
 from pydantic import BaseModel, Extra, PrivateAttr
 from pydantic.fields import Undefined
+from pydantic.generics import GenericModel
+
+skip_36 = pytest.mark.skipif(sys.version_info < (3, 7), reason='generics only supported for python 3.7 and above')
 
 
 def test_private_attribute():
@@ -180,3 +184,19 @@ class Config:
     m = MyModel(x='hello')
     assert m.dict() == {'x': 'hello'}
     assert m._private_attr == 123
+
+
+@skip_36
+def test_generic_private_attribute():
+    T = TypeVar('T')
+
+    class Model(GenericModel, Generic[T]):
+        value: T
+        _private_value: T
+
+        class Config:
+            underscore_attrs_are_private = True
+
+    m = Model[int](value=1, _private_attr=3)
+    m._private_value = 3
+    assert m.dict() == {'value': 1}