[Python] Pickle works but fails pytest

I've created a module persist containing a function obj_pickle which I'm using to pickle various objects in my project. It work fine but it's failing pytest, returning;

> pickle.dump(obj, file_handle, protocol=protocol)
E AttributeError: Can't pickle local object 'test_object.<locals>.TestObject'


Running:

• python 3.12
• pytest Version: 8.3.5

I see that the are similar looking issues tied to multiprocessing but I don't think pytest is exposing that issue. Code below, and many thanks.

# lib.persist.py
from pathlib import Path
import pickle


def obj_pickle(obj: object, dirath, protocol: int = pickle.HIGHEST_PROTOCOL) -> None:
"""
Pickle an object to a byte file.
"""
if not dir.exists():
dir.mkdir(parents=True, exist_ok=True)
path = Path(dir, obj.instance_name + '.pkl')
with open(path, "wb") as file_handle:
pickle.dump(obj, file_handle, protocol=protocol)
print(f"{obj.__class__.__name__} object {obj.instance_name} saved to {path}")


# tests.test_persist.py
from pathlib import Path

import pytest

from lib.persist import obj_pickle

TEST_DIR = Path("test_dir")


@pytest.fixture
def test_object():
class TestObject():
def __init__(self, instance_file_name):
self.instance_file_name = instance_file_name
self.data = "This is a test object."

test_object = TestObject("test_object_file")

return test_object


def test_obj_pickle(test_object):
obj_pickle(test_object, Path(TEST_DIR))
path = Path(TEST_DIR, "test_object_file" + ".pkl")
assert path.exists()

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