[BUG] Python-Flask Models do not Validate on Initialization

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

Generated model classes in Python-Flask have an __init__() method that assigns constructor arguments directly to the ._attribute, instead of assigning to the .attribute (which would implicitly call the associated setter method), and this therefore bypasses validation. Bypassing validation at object initialization time is undesirable.

E.g.

If one has a schema like the following:

note:
  type: object
  description: A note
  properties:
    type:
      description: The type of the note
      type: string
      enum:
        - "to-do"
        - "reminder"
    text:
      description: The content of the note
      type: string
  required:
    - type
    - text
  example:
    text: "Get milk at the store."
    type: "to-do"

then the Note model generated from that (in python-flask mode) can be imported and used like this:

>>> from openapi_server.models.note import Note
>>> note = Note()
>>> # Notice the lack of exception despite not having all the required parameters set

which is undesired behavior. The Note() model should validate on initialization, and the above expression should result in an exception (due to missing required properties).

openapi-generator version

5.2.1

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  version: 1.0.0
  title: Example note API
paths:
  /note:
    get:
      summary: Get note
      responses:
        '200':
          description: Success
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Note'
components:
  schemas:
    Note:
      type: object
      description: A clinical note
      properties:
        text:
          description: The content of the note
          type: string
        type:
          description: The note type
          type: string
          enum:
            - "to-do"
            - "reminder"
      required:
        - text
        - type

Generation Details

openapi-generator-cli generate -g python-flask -o server -i openapi.yaml

(openapi.yaml is a file containing the above specification)

Steps to reproduce

1. openapi-generator-cli generate -g python-flask -o server -i openapi.yaml
2. cd server
3. python

>>> from openapi_server.models.note import Note
>>> Note()  # This is an invalid note (no `text` or `type`) and this call *should* result in an exception
    {'text': None, 'type': None}

The last output line above should be an error. Notice the desired behavior (validation) does happen when changing the value of an existing Note:

>>> note = Note()
>>> note.type = 'asdf'  # Not an allowed value, see the enum in the spec
    Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/connor/Documents/tmp/server/openapi_server/models/note.py", line 97, in type
    raise ValueError(
ValueError: Invalid value for `type` (asdf), must be one of ['to-do', 'reminder']

Related issues/PRs

Suggest a fix

This bug should be fixable by changing this line of the template.

self._{{name}} = {{name}}

should be

self.{{name}} = {{name}}

this will ensure that every assigned value gets passed through the setter for that attribute, now including at initialization time.

Start with modules/openapi-generator/src/main/resources/python-flask/model.mustache, especially the constructor assignment described in the issue. Generate the provided OpenAPI example with the python-flask command and reproduce Note() and invalid enum assignment behavior. Done means initialization validates required and enum properties consistently with later attribute assignments.