Repository: samuelcolvin/pydantic (python)

Issue:
PrivateAttr causes failing type check
### Checks

- [x] I added a descriptive title to this issue
- [x] I have searched (google, github) for similar issues and couldn't find anything
- [x] I have read and followed [the docs](https://pydantic-docs.helpmanual.io/) and still think this is a bug

# Bug

Output of `python -c "import pydantic.utils; print(pydantic.utils.version_info())"`:

```
        pydantic version: 1.7
       pydantic compiled: True
            install path: /Users/ahedges/.pyenv/versions/3.7.8/envs/sdf/lib/python3.7/site-packages/pydantic
          python version: 3.7.8 (default, Sep 16 2020, 18:33:23)  [Clang 11.0.3 (clang-1103.0.32.59)]
                platform: Darwin-19.6.0-x86_64-i386-64bit
optional deps. installed: ['typing-extensions']
```

I created a file with the following code:

```py
from pydantic import BaseModel, Field, PrivateAttr

class TestObject(BaseModel):
    public_field: str = Field()
    _private_field: str = PrivateAttr()
```

I then ran mypy (version 0.790) using the pydantic plugin. I received the following type error:

```
test.py:5: error: Incompatible types in assignment (expression has type "PrivateAttr", variable has type "str")
Found 1 error in 1 file (checked 1 source file)
```

`PrivateAttr` should likely not cause a type error here, similarly to how `Field` works.

Pull request description:
Refactor PrivateAttr to type-check like Field
## Change Summary

- Fix mypy assignment error when using `PrivateAttr`
  - `PrivateAttr` is renamed `ModelPrivateAttr` and modeled after `ModelField`
  - A `PrivateAttr` function now exists, modeled after `Field`
- Documentation and error message are now more consistent between `PrivateAttr` and `Field`
- Made changes to typing where needed

I did not add any unit tests because I'm unsure how to write a unit test for preventing mypy errors.

## Related issue number

Fixes #2048.

## Checklist

* [x] Unit tests for the changes exist
* [x] Tests pass on CI and coverage remains at 100%
* [x] Documentation reflects the changes where applicable
* [x] `changes/<pull request or issue id>-<github username>.md` file added describing change


Patch:
diff --git a/changes/2048-aphedges.md b/changes/2048-aphedges.md
new file mode 100644
index 000000000..0407f2367
--- /dev/null
+++ b/changes/2048-aphedges.md
@@ -0,0 +1,1 @@
+Fix mypy assignment error when using `PrivateAttr`
diff --git a/pydantic/fields.py b/pydantic/fields.py
index c451770c0..36ea690a2 100644
--- a/pydantic/fields.py
+++ b/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,
+    )
diff --git a/pydantic/main.py b/pydantic/main.py
index bc5444a70..bc0b5be5d 100644
--- a/pydantic/main.py
+++ b/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; '


Test patch:
diff --git a/tests/mypy/modules/success.py b/tests/mypy/modules/success.py
index 202184c93..3d48a33e1 100644
--- a/tests/mypy/modules/success.py
+++ b/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()
diff --git a/tests/test_private_attributes.py b/tests/test_private_attributes.py
index 07f702c7f..962145229 100644
--- a/tests/test_private_attributes.py
+++ b/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)