[BUG] [Python-flask] Enum value validation occur 500 error on stub server

First of all, I want to thank all contributors!

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

If a property has an enum setting and is not nullable, API gets 500 error.

requestBody:
  content:
    application/json:
      schema:
        type: object
        properties:
          foo_id:
            type: string               
            enum:
              - 'foo'
              - 'bar'

openapi-generator version

6.2.0

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  description: Appreciate so much contributors
  version: 1.0.0
  title: Hello OpenAPI
  contact:
    email: shimizu@thedott.io
tags:
  - name: Tag
    description: the Tag
paths:
  /foobar/{foo_id}:
    get:
      tags:
        - Tag
      summary: foo
      description: bar
      operationId: foobar_get
      parameters:
        - in: path
          name: foo_id
          schema:
            type: string
            enum:
              - 'foo'
              - 'bar'
          required: true
      responses:
        '200':
          description: foobar
  /foobar:
    post:
      tags:
        - Tag
      summary: foo
      description: bar
      operationId: foobar_post
      requestBody:
        content:
          application/json:
            schema:
              type: object
              properties:
                foo_id:
                  type: string               
                  enum:
                    - 'foo'
                    - 'bar'
      responses:
        '200':
          description: foobar

Generation Details

openapi-generator generate -i swagger-build.yaml -g python-flask

Python 3.8.12

Steps to reproduce

1. Make a parameter with an enum and not nullable in RequestBody
2. Generate stub server
3. Post a correct param to the API
4. Got 500 error

Related issues/PRs

Suggest a fix

It worked in my 5.3.1 generator.

I think the problem is in __init__ function in Autogenerated Model class.

In 5.3.1, member variables in the init function were private (like self._foobar) but public (self.foobar) in 6.2.0.
This change causes a problem.

The setter is called forcibly when assigning some variable to a public member like as following sample:

class Foo():
  def __init__(self, foo_id=None):
    self.foo_id = foo_id  ## Setter called (6.2.0)
    self._foo_id = foo_id  ## Setter doesn't call (5.3.1)
  
  @property
  def foo_id(self):
    return self._foo_id

  @foo.setter
  def foo_id(self, foo_id):
    print('Setter called')
    self._foo_id = foo_id

The detailed problem is in the deserialize_model function that excuted by using `Model.from_dict() function.

def deserialize_model(data, klass):
    """Deserializes list or dict to model.

    :param data: dict, list.
    :type data: dict | list
    :param klass: class literal.
    :return: model object.
    """
    instance = klass()

    if not instance.openapi_types:
        return data

    for attr, attr_type in six.iteritems(instance.openapi_types):
        if data is not None \
                and instance.attribute_map[attr] in data \
                and isinstance(data, (list, dict)):
            value = data[instance.attribute_map[attr]]
            setattr(instance, attr, _deserialize(value, attr_type))

    return instance

The class is called without any parameters in instance = klass() and will cause a 500.
In this moment, the param is None and raise ValueError.

Invalid value for `foo_id` (None), must be one of ['foo', 'bar']

I understand the problem, ... but I don't know how it should be so I cannot make any PR.
I'm using nullable: true setting for the workaround this time.

Start by generating a python-flask stub from the OpenAPI declaration and reproduce the 500 error with the enum request body. Read the generated model class's init function and the deserialize_model function, focusing on how None is handled before enum validation. Done means a valid request no longer triggers a 500 error while invalid enum values remain validated.