allow for shallow copies

changes/4093-timkpaine.md

@@ -0,0 +1,2 @@
+Allow for shallow copies of attributes, adjusting the behavior of #3642
+`Config.copy_on_model_validation` is now a str enum of `["", "deep", "shallow"]` corresponding to reference, deep copy, shallow copy.

pydantic/config.py

@@ -36,6 +36,12 @@ class Extra(str, Enum):
     forbid = 'forbid'
 
 
+class Copy(str, Enum):
+    none = ''
+    deep = 'deep'
+    shallow = 'shallow'
+
+
 class BaseConfig:
     title: Optional[str] = None
     anystr_lower: bool = False
@@ -62,8 +68,10 @@ class BaseConfig:
     json_encoders: Dict[Union[Type[Any], str, ForwardRef], AnyCallable] = {}
     underscore_attrs_are_private: bool = False
 
-    # whether inherited models as fields should be reconstructed as base model
-    copy_on_model_validation: bool = True
+    # whether inherited models as fields should be reconstructed as base model,
+    # and whether such a copy should be shallow or deep
+    copy_on_model_validation: Copy = Copy.deep
+
     # whether `Union` should check all allowed types before even trying to coerce
     smart_union: bool = False
 

pydantic/main.py

@@ -25,7 +25,7 @@
 )
 
 from .class_validators import ValidatorGroup, extract_root_validators, extract_validators, inherit_validators
-from .config import BaseConfig, Extra, inherit_config, prepare_config
+from .config import BaseConfig, Copy, Extra, inherit_config, prepare_config
 from .error_wrappers import ErrorWrapper, ValidationError
 from .errors import ConfigError, DictError, ExtraError, MissingError
 from .fields import MAPPING_LIKE_SHAPES, Field, FieldInfo, ModelField, ModelPrivateAttr, PrivateAttr, Undefined
@@ -675,7 +675,9 @@ def __get_validators__(cls) -> 'CallableGenerator':
     @classmethod
     def validate(cls: Type['Model'], value: Any) -> 'Model':
         if isinstance(value, cls):
-            if cls.__config__.copy_on_model_validation:
+            if cls.__config__.copy_on_model_validation == Copy.shallow:
+                return value._copy_and_set_values(value.__dict__, value.__fields_set__, deep=False)
+            elif cls.__config__.copy_on_model_validation:
                 return value._copy_and_set_values(value.__dict__, value.__fields_set__, deep=True)
             else:
                 return value

tests/test_main.py

@@ -1567,6 +1567,28 @@ class Config:
     assert t.dict() == {'id': '1234567890', 'user': {'id': 42, 'hobbies': ['scuba diving']}}
 
 
+def test_model_exclude_copy_on_model_validation_shallow():
+    """When `Config.copy_on_model_validation` is set and `Config.copy_on_model_validation_shallow` is set,
+    do the same as the previous test but perform a shallow copy"""
+
+    class User(BaseModel):
+        class Config:
+            copy_on_model_validation = 'shallow'
+
+        hobbies: List[str]
+
+    my_user = User(hobbies=['scuba diving'])
+
+    class Transaction(BaseModel):
+        user: User = Field(...)
+
+    t = Transaction(
+        user=my_user,
+    )
+
+    assert t.user.hobbies is my_user.hobbies  # unlike above, this should be a shallow copy
+
+
 def test_validation_deep_copy():
     """By default, Config.copy_on_model_validation should do a deep copy"""