Cannot use model_copy(deep=True) on a model instance which was created using model_construct, when there is a model_post_init in the class hierarchy.

[babygrimes]

Initial Checks

• I confirm that I'm using Pydantic V2

Description

This standalone code produces the issue. When constructing a model instance using .model_construct instead of .model_validate or __init__, and if the model class defines model_post_init (on the class, or earlier in the hierarchy - both will trigger the issue), the below error is produced.

If the m.model_copy(deep=True) line is changed to m.model_validate(m, from_attributes=True, strict=True) (just to show the difference, and is what I'm currently using as a workaround) - everything appears to work as expected.

Additionally, you can comment out the model_post_init in the example below, and everything again works as expected (though I really want to use model_post_init in my real code for various reasons).

I traced this as far as I could into main.py:model_construct and it appears to be related to lines 238-249. It appears in the model_construct path there is no initialization of __pydantic_private__, resulting in the stack trace here:

  File "/home/dave/test_deepcopy.py", line 21, in <module>
    main()
  File "/home/dave/test_deepcopy.py", line 17, in main
    m.model_copy(deep=True)
  File "/home/dave/axial/src/services/axial-api.git/venv/lib/python3.11/site-packages/pydantic/main.py", line 266, in model_copy
    copied = self.__deepcopy__() if deep else self.__copy__()
             ^^^^^^^^^^^^^^^^^^^
  File "/home/dave/axial/src/services/axial-api.git/venv/lib/python3.11/site-packages/pydantic/main.py", line 723, in __deepcopy__
    if self.__pydantic_private__ is None:
       ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/dave/axial/src/services/axial-api.git/venv/lib/python3.11/site-packages/pydantic/main.py", line 764, in __getattr__
    return super().__getattribute__(item)  # Raises AttributeError if appropriate
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'Model' object has no attribute '__pydantic_private__'. Did you mean: '__pydantic_complete__'?```



### Example Code

```Python
#!/usr/bin/env python3
# coding: utf-8
from typing import Any

import pydantic


class Model(pydantic.BaseModel, revalidate_instances="always"):
    id: int  # noqa: A003

    def model_post_init(self, context: Any) -> None:
        super().model_post_init(context)


def main() -> None:
    m = Model.model_construct({"id": 1})
    m.model_copy(deep=True)


if __name__ == "__main__":
    main()

Python, Pydantic & OS Version

pydantic version: 2.6.4
        pydantic-core version: 2.16.3
          pydantic-core build: profile=release pgo=true
                 install path: /home/dave/axial/src/services/axial-api.git/venv/lib/python3.11/site-packages/pydantic
               python version: 3.11.8 (main, Feb 25 2024, 16:41:26) [GCC 9.4.0]
                     platform: Linux-5.15.0-1056-aws-x86_64-with-glibc2.31
             related packages: fastapi-0.110.0 typing_extensions-4.10.0 mypy-1.9.0
                       commit: unknown

[sydney-runkle]

Looks like a bug! Thanks for reporting this!

[babygrimes]

It occurs to me that maybe this simple patch would be sufficient to fix this. The thinking is that __pydantic_private__ is allowed to be any of:

0. An unset attribute
1. None
2. Dictionary of private attributes

The issue that is triggering the bug is that it is unset when the __deepcopy__ call is made, but the internals assume it is either None/Dict. This patch adds coverage of "Unset" (pass).

(venv) [dmgrime@dave-laptop:~]$ diff -u venv/lib/python3.11/site-packages/pydantic/main.py /tmp
--- venv/lib/python3.11/site-packages/pydantic/main.py  2024-04-03 09:41:34.015680649 -0400
+++ /tmp/main.py        2024-04-03 09:40:54.425678622 -0400
@@ -720,7 +720,9 @@
         # and attempting a deepcopy would be marginally slower.
         _object_setattr(m, '__pydantic_fields_set__', copy(self.__pydantic_fields_set__))

-        if self.__pydantic_private__ is None:
+        if not hasattr(self, "__pydantic_private__"):
+            pass
+        elif self.__pydantic_private__ is None:
             _object_setattr(m, '__pydantic_private__', None)
         else:
             _object_setattr(
(venv) [dmgrime@dave-laptop:~]$

[sydney-runkle]

@babygrimes,

Please open a PR, I'd be happy to review!

[babygrimes]

Will get you a PR today!