[BUG][Python] model template copy refers to new_cls (undefined)

Created by: tuanchien

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

The Python generator template's __copy__ function refers to new_cls, which is not defined in the function.

See: https://github.com/OpenAPITools/openapi-generator/blob/9464999d9c799ff9eb723886206ea98b7222032d/modules/openapi-generator/src/main/resources/python/model_templates/methods_shared.mustache#L22

This propagates into the generated model_utils.py file. It will result in failed operations if you try to copy() an OpenApiModel object.

openapi-generator version

This is present in the master branch but also 5.3.0

OpenAPI declaration file content or url

This is reproducible using the petstore sample already present in the repo.

Generation Details

None. Use petstore sample.

Steps to reproduce

From the openapi-generator/samples/client/petstore/python directory, launch Python and run

import petstore_api
from petstore_api.model.class_model import ClassModel
import copy

a = ClassModel()
b = copy.deepcopy(a)  # OK
c = copy.copy(a)  # Raises an error

It produces an error like:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.8/copy.py", line 84, in copy
    return copier(x)
  File "/home/user/openapi-generator/samples/client/petstore/python/petstore_api/model_utils.py", line 195, in __copy__
    return new_cls.__new__(cls, **self.__dict__)
NameError: name 'new_cls' is not defined

Related issues/PRs

Suggest a fix

Change https://github.com/OpenAPITools/openapi-generator/blob/9464999d9c799ff9eb723886206ea98b7222032d/modules/openapi-generator/src/main/resources/python/model_templates/methods_shared.mustache#L22 to

            return cls.__new__(cls, **self.__dict__)

[BUG][Python] model template __copy__ refers to new_cls (undefined)