[BUG] [python-nextgen] (De)serialization of enums fails due to missing conversion methods

Created by: TimoGuenther

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-nextgen generator assumes the existence of (from|to)_(dict|json) methods on generated enums, which causes enum (de)serialization to fail with an exception.

Affected:

singular allOf: The parent expects (from|to)_dict from the enum.
anyOf, oneOf, plural allOf: The composite model expects (from|to)_(dict|json) from the wrapped enum(s).

Impact:

That is how DRF Spectacular generates enums, for example.

openapi-generator version

6.3.0-20230125.125049-89

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: ""
  version: ""
paths: {}
components:
  schemas:
    Parent:
      type: object
      properties:
        number:
          allOf:
          - $ref: '#/components/schemas/Number'
    Number:
      enum:
      - one
      - two
      - three
      type: string

Generation Details

java -jar openapi-generator-cli-6.3.0-20230125.125049-89.jar generate -i openapi-schema.yaml -g python-nextgen

Steps to reproduce

Assuming the import problem described in #14523 is fixed:

>>> from openapi_client import Parent, Number
>>> Parent.from_dict(dict(number="one"))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "openapi_client/models/parent.py", line 73, in from_dict
    "number": Number.from_dict(obj.get("number")) if obj.get("number") is not None else None
AttributeError: type object 'Number' has no attribute 'from_dict'
>>> Parent(number=Number.ONE).to_dict()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "openapi_client/models/parent.py", line 60, in to_dict
    _dict['number'] = self.number.to_dict()
AttributeError: 'Number' object has no attribute 'to_dict'

Related issues/PRs

#14523

Suggest a fix

Manually replacing the from_dict call with an enum value lookup (by calling the enum class directly) and removing the block that calls to_dict on the enum from parent.py fixes (de)serialization for singular allOf:

>>> from openapi_client import Parent, Number
>>> Parent.from_dict(dict(number="one"))
Parent(number=<Number.ONE: 'one'>)
>>> Parent(number=Number.ONE).to_dict()
{'number': <Number.ONE: 'one'>}

This must be automated.

The fix for composite models seems more involved (e.g., checking for existence of the required methods on the wrapped value).

Alternatively, the enums could feature trivial implementations of the required methods.