[BUG][python] deepcopy of model gives KeyError: '_data_store'

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?
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Description

I ran into this as I was attempting to use dataclasses.asdict(obj). I tracked it down to use of a deepcopy call. While copying the openapi model object, I believe the default deepcopy is missing the object attributes.

openapi-generator version

master

OpenAPI declaration file content or url

I used the sample petstore api in: samples/openapi3/client/petstore/python

Generation Details

N/A

Steps to reproduce

from copy import deepcopy
from petstore_api.model.banana import Banana
first = Banana(5.2)
second = deepcopy(first)

<snip>
KeyError: '_data_store'

Related issues/PRs

Suggest a fix

I came up with a solution to this by updating the model_utils.py, specifically the OpenApiModel, adding:

    def __copy__(self):
        cls = self.__class__
        result = cls.__new__(cls)
        result.__dict__.update(self.__dict__)
        return result

    def __deepcopy__(self, memo):
        cls = self.__class__
        result = cls.__new__(cls)
        for k, v in self.__dict__.items():
            setattr(result, k, deepcopy(v, memo))
        return result

Note that this worked only for a Normal object without a discriminator. I ran into a separate issue (might be unrelated?), where the same test with a discriminator gave the following:

from copy import deepcopy
from petstore_api.model.mammal import Mammal
deeepcopy(Mammal())

<snip>
petstore_api.exceptions.ApiValueError: Cannot deserialize input data due to missing discriminator. The discriminator property 'className' is missing at path: ()

Could there be other pitfalls with implementation of the deepcopy above?

Start with samples/openapi3/client/petstore/python and model_utils.py, focusing on OpenApiModel and the Banana deepcopy reproduction. Compare deepcopy(Banana(5.2)) with deepcopy(Mammal()) and verify the fix avoids the '_data_store' KeyError while accounting for the reported discriminator failure.