Refactor PrivateAttr to type-check like Field (#2057)

* Refactor PrivateAttr to type-check like Field

* Change TypeError to ValueError for consistency

* Add PrivateAttr example to mypy tests

changes/2048-aphedges.md

@@ -0,0 +1 @@
+Fix mypy assignment error when using `PrivateAttr`

pydantic/fields.py

@@ -799,21 +799,10 @@ class ModelField(Representation):
         return args
 
 
-class PrivateAttr(Representation):
-    """
-    Indicates that attribute is only used internally and never mixed with regular fields.
-
-    Types or values of private attrs are not checked by pydantic and it's up to you to keep them relevant.
-
-    Private attrs are stored in model __slots__.
-    """
-
+class ModelPrivateAttr(Representation):
     __slots__ = ('default', 'default_factory')
 
     def __init__(self, default: Any = Undefined, *, default_factory: Optional[NoArgAnyCallable] = None) -> None:
-        if default is not Undefined and default_factory is not None:
-            raise TypeError('default and default_factory args can not be used together')
-
         self.default = default
         self.default_factory = default_factory
 
@@ -825,3 +814,28 @@ class PrivateAttr(Representation):
             other.default,
             other.default_factory,
         )
+
+
+def PrivateAttr(
+    default: Any = Undefined,
+    *,
+    default_factory: Optional[NoArgAnyCallable] = None,
+) -> Any:
+    """
+    Indicates that attribute is only used internally and never mixed with regular fields.
+
+    Types or values of private attrs are not checked by pydantic and it's up to you to keep them relevant.
+
+    Private attrs are stored in model __slots__.
+
+    :param default: the attribute’s default value
+    :param default_factory: callable that will be called when a default value is needed for this attribute
+      If both `default` and `default_factory` are set, an error is raised.
+    """
+    if default is not Undefined and default_factory is not None:
+        raise ValueError('cannot specify both default and default_factory')
+
+    return ModelPrivateAttr(
+        default,
+        default_factory=default_factory,
+    )

pydantic/main.py

@@ -29,7 +29,7 @@ from typing import (
 from .class_validators import ValidatorGroup, extract_root_validators, extract_validators, inherit_validators
 from .error_wrappers import ErrorWrapper, ValidationError
 from .errors import ConfigError, DictError, ExtraError, MissingError
-from .fields import SHAPE_MAPPING, ModelField, PrivateAttr, Undefined
+from .fields import SHAPE_MAPPING, ModelField, ModelPrivateAttr, PrivateAttr, Undefined
 from .json import custom_pydantic_encoder, pydantic_encoder
 from .parse import Protocol, load_file, load_str_bytes
 from .schema import default_ref_template, model_schema
@@ -215,7 +215,7 @@ class ModelMetaclass(ABCMeta):
         validators: 'ValidatorListDict' = {}
 
         pre_root_validators, post_root_validators = [], []
-        private_attributes: Dict[str, PrivateAttr] = {}
+        private_attributes: Dict[str, ModelPrivateAttr] = {}
         slots: Set[str] = namespace.get('__slots__', ())
         slots = {slots} if isinstance(slots, str) else set(slots)
 
@@ -271,7 +271,7 @@ class ModelMetaclass(ABCMeta):
 
             for var_name, value in namespace.items():
                 can_be_changed = var_name not in class_vars and not isinstance(value, untouched_types)
-                if isinstance(value, PrivateAttr):
+                if isinstance(value, ModelPrivateAttr):
                     if not is_valid_private_name(var_name):
                         raise NameError(
                             f'Private attributes "{var_name}" must not be a valid field name; '

tests/mypy/modules/success.py

@@ -9,7 +9,7 @@ from datetime import date, datetime
 from typing import Any, Dict, Generic, List, Optional, TypeVar
 
 from pydantic import BaseModel, NoneStr, PyObject, StrictBool, root_validator, validate_arguments, validator
-from pydantic.fields import Field
+from pydantic.fields import Field, PrivateAttr
 from pydantic.generics import GenericModel
 from pydantic.typing import ForwardRef
 
@@ -145,3 +145,7 @@ class MyConf(BaseModel):
 conf = MyConf()
 var1: date = conf.str_pyobject(2020, 12, 20)
 var2: date = conf.callable_pyobject(2111, 1, 1)
+
+
+class MyPrivateAttr(BaseModel):
+    _private_field: str = PrivateAttr()

tests/test_private_attributes.py

@@ -158,5 +158,5 @@ def test_slots_are_ignored():
 
 
 def test_default_and_default_factory_used_error():
-    with pytest.raises(TypeError, match='default and default_factory args can not be used together'):
+    with pytest.raises(ValueError, match='cannot specify both default and default_factory'):
         PrivateAttr(default=123, default_factory=lambda: 321)