Cloudpickle is broken in Pydantic 2.11.0

[dreadatour]

After this change in pydantic-core: pydantic/pydantic-core#1616

How to reproduce:

model-write.py:

from cloudpickle import dumps
from pydantic import BaseModel

class MyModel(BaseModel):
    name: str = ""

serialized = dumps(MyModel)

with open("model.pkl", "wb") as f:
    f.write(serialized)

model-read.py

from cloudpickle import loads

with open("model.pkl", "rb") as f:
    serialized = f.read()

MyModel = loads(serialized)
model = MyModel(name="bar")

assert model.name == "bar"

$ uv pip install cloudpickle pydantic
Using Python 3.13.2 environment at: /Users/vlad/.virtualenvs/test-pydantic
Resolved 6 packages in 131ms
Installed 6 packages in 5ms
 + annotated-types==0.7.0
 + cloudpickle==3.1.1
 + pydantic==2.11.1
 + pydantic-core==2.33.0
 + typing-extensions==4.13.0
 + typing-inspection==0.4.0
$ python model-write.py
$ python model-read.py
Traceback (most recent call last):
  File "/Users/vlad/playground/model-read.py", line 9, in <module>
    assert model.name == "bar"
           ^^^^^^^^^^
  File "/Users/vlad/.virtualenvs/test-pydantic/lib/python3.13/site-packages/pydantic/main.py", line 984, in __getattr__
    raise AttributeError(f'{type(self).__name__!r} object has no attribute {item!r}')
AttributeError: 'MyModel' object has no attribute 'name'
$

[dreadatour]

Related to #1002

[dreadatour]

It looks like the issue is with references now used for optimizations in Rust code in pydantic-core (here).

MyModel.__pydantic_validator__ before serialization:

SchemaValidator(title="MyModel", validator=Model(
    ModelValidator {
        revalidate: Never,
        validator: ModelFields(
            ModelFieldsValidator {
                fields: [
                    Field {
                        name: "name",
                        lookup_key_collection: LookupKeyCollection {
                            by_name: Simple(
                                LookupPath {
                                    first_item: PathItemString {
                                        key: "name",
                                        py_key: Py(
                                            0x0000000101d1bd50,
                                        ),
                                    },
                                    rest: [],
                                },
                            ),
                            by_alias: None,
                            by_alias_then_name: None,
                        },
                        name_py: Py(
                            0x00000001013da278,
                        ),
                        validator: WithDefault(
                            WithDefaultValidator {
                                default: Default(
                                    Py(
                                        0x00000001013d33d8,
                                    ),
                                ),
                                on_error: Raise,
                                validator: Str(
                                    StrValidator {
                                        strict: false,
                                        coerce_numbers_to_str: false,
                                    },
                                ),
                                validate_default: false,
                                copy_default: false,
                                name: "default[str]",
                                undefined: Py(
                                    0x00000001009f4730,
                                ),
                            },
                        ),
                        frozen: false,
                    },
                ],
                model_name: "MyModel",
                extra_behavior: Ignore,
                extras_validator: None,
                extras_keys_validator: None,
                strict: false,
                from_attributes: false,
                loc_by_alias: true,
                validate_by_alias: None,
                validate_by_name: None,
            },
        ),
        class: Py(
            0x000000011f61bd10,
        ),
        generic_origin: None,
        post_init: None,
        frozen: false,
        custom_init: false,
        root_model: false,
        undefined: Py(
            0x00000001009f4730,
        ),
        name: "MyModel",
    },
), definitions=[], cache_strings=True)

MyModel.__pydantic_validator__ after deserialization:

SchemaValidator(title="MyModel", validator=Prebuilt(
    PrebuiltValidator {
        schema_validator: Py(
            0x00000001520eddc0,
        ),
    },
), definitions=[], cache_strings=True)

Naive approaches with using register_pickle_by_value does not works.

1. pydantic_core only:

import cloudpickle
import pydantic_core

cloudpickle.register_pickle_by_value(pydantic_core)

Gives this error on deserialization:

Traceback (most recent call last):
  File "/Users/vlad/playground/model-read.py", line 6, in <module>
    MyModel = cloudpickle.loads(serialized)
AttributeError: type object 'SchemaSerializer' has no attribute 'to_json'

2. Both pydantic and pydantic_core:

import cloudpickle
import pydantic
import pydantic_core

cloudpickle.register_pickle_by_value(pydantic)
cloudpickle.register_pickle_by_value(pydantic_core)

Gives this error on serialization:

Traceback (most recent call last):
  File "/Users/vlad/playground/model-write.py", line 15, in <module>
    serialized = cloudpickle.dumps(MyModel)
  File "/Users/vlad/.virtualenvs/test-pydantic/lib/python3.13/site-packages/cloudpickle/cloudpickle.py", line 1537, in dumps
    cp.dump(obj)
    ~~~~~~~^^^^^
  File "/Users/vlad/.virtualenvs/test-pydantic/lib/python3.13/site-packages/cloudpickle/cloudpickle.py", line 1303, in dump
    return super().dump(obj)
           ~~~~~~~~~~~~^^^^^
TypeError: cannot pickle 'classmethod_descriptor' object

(related issue: cloudpipe/cloudpickle#486)

[dreadatour]

Have no ideas left on can we solve it on our side.

Created an issue in cloudpickle repo: cloudpipe/cloudpickle#565

[dmpetrov]

is it still an issue? can we close it?

[dreadatour]

is it still an issue? can we close it?

Let me check if this was fixed and we can close it by updating dependencies.