[REQ] python-flask enums needs allowable_values as per other python types

Created by: Johnlon

Se https://github.com/OpenAPITools/openapi-generator/pull/10954

We need a collection of allowable_values on the generated flask enum classes so that we can validate the string values before transmitting them.

allowable_values = [ HAPPPY, SAD ]

Describe the solution you'd like

A product like this the below that carries the allowable values and a utility function 'of' for validation. Notice '######### ADDITIONAL LINES START' and '######### ADDITIONAL LINES END'

# coding: utf-8

from __future__ import absolute_import
from datetime import date, datetime  # noqa: F401

from typing import List, Dict  # noqa: F401

from petstore.models.base_model_ import Model
from petstore import util


class MyEnum(Model):
    """NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).

    Do not edit the class manually.
    """

    """
    allowed enum values
    """
    HAPPY = "happy"
    SAD = "sad"

    ######### ADDITIONAL LINES START
    allowable_values = [HAPPY, SAD]  # noqa: E501
    enum_items = {  "HAPPY" : HAPPY,  "SAD" : SAD }  # noqa: E501

    @staticmethod
    def of(value):
        """
        return value if it is a valid enum value or none
        """
        if not value or value in MyEnum.allowable_values:
            return value

        raise ValueError(
            "Invalid enum value for `MyEnum` ({0}), must be one of {1}"  # noqa: E501
            .format(value, MyEnum.allowable_values)
        )
    ######### ADDITIONAL LINES END

    def __init__(self):  # noqa: E501
        """MyEnum - a model defined in OpenAPI

        """
        self.openapi_types = {
        }

        self.attribute_map = {
        }

    @classmethod
    def from_dict(cls, dikt) -> 'MyEnum':
        """Returns the dict as a model

        :param dikt: A dict.
        :type: dict
        :return: The MyEnum of this MyEnum.  # noqa: E501
        :rtype: MyEnum
        """
        return util.deserialize_model(dikt, cls)

Which allows a usage like ....

    candidate_value = "happy"
    some_coms_field = MyEnum.of(candidate_value)

.. so that we get a validation of the value 'candidate_value' before it is serialised to JSON

Describe alternatives you've considered

None

Additional context

Many other variants have 'allowable_values'