[BUG] Unable to unpickle an object in python

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Can't unpickle pickled model in python. On unpickling it throws a KeyError. Expect it to be unpickled correctly.

openapi-generator version

5.2.1

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  description: test
  version: 1.0.1-1
  title: test
  contact:
    name: Pavel Safronov
    email: pv.safronov@gmail.com
  license:
    name: Apache 2.0
    url: 'http://www.apache.org/licenses/LICENSE-2.0.html'
components:
  schemas:
    Test:
      required:
        - test
      properties:
        test:
          type: string
paths: {}

servers:
  - url: 'https://test.com'

Generation Details

Something like that. So pretty much regular generation

wget https://repo1.maven.org/maven2/org/openapitools/openapi-generator-cli/5.2.1/openapi-generator-cli-5.2.1.jar -O openapi-generator-cli.jar

java \
    -jar $tmp_dir/openapi-generator-cli.jar \
    generate \
    -g python \
    -i ${swaggerfile} \
    --package-name testopenapi \
    -o ${out_dir}

Steps to reproduce

In [8]: import pickle                                      

In [9]: from testopenapi.model.test import Test            

In [10]: test = Test(test="test")                          

In [11]: pickle.loads(pickle.dumps(test))                  
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-11-b386bc64ce4d> in <module>
----> 1 pickle.loads(pickle.dumps(test))

~/git/github.com/prius/leetcode-swagger/python/out/testopenapi/model_utils.py in __getattr__(self, attr)
    186     def __getattr__(self, attr):
    187         """get the value of an attribute using dot notation: `instance.attr`"""
--> 188         return self.__getitem__(attr)
    189 
    190     def __new__(cls, *args, **kwargs):

~/git/github.com/prius/leetcode-swagger/python/out/testopenapi/model_utils.py in __getitem__(self, name)
    493     def __getitem__(self, name):
    494         """get the value of an attribute using square-bracket notation: `instance[attr]`"""
--> 495         if name in self:
    496             return self.get(name)
    497 

~/git/github.com/prius/leetcode-swagger/python/out/testopenapi/model_utils.py in __contains__(self, name)
    507             return name in self.__dict__
    508 
--> 509         return name in self.__dict__['_data_store']
    510 
    511     def to_dict(self):

KeyError: '_data_store'

I see the same behavior in the real project as well.

Related issues/PRs

Looks like there is something similar going on here https://github.com/OpenAPITools/openapi-generator/issues/9669

However not sure it is fixable the same way, so file a bug anyway.

Suggest a fix

The failure is in generated testopenapi/model_utils.py, where unpickling reaches getattr, getitem, and contains before _data_store exists. Reproduce the provided pickle round trip from the OpenAPI spec, trace the generated Python model behavior, and verify that pickle.loads(pickle.dumps(test)) completes without a KeyError.