[Python] Multilevel inheritance discriminator support

[eak24]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used? 4.2.3
• Have you search for related issues/PRs? Yes - they are [Python] Adds allOf/oneOf/anyOf composed models #4446
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

@spacether

When using OAS to describe a complex polymorphic graph, the client is currently unable to serialize grandchildren of any given type (subclasses of subclasses.) I have made a recursive model lookup function, which recursively looks at the discriminators of the subclasses (discriminated classes) until the leaf nodes, but that has very bad performance, reducing performance to 1/100 of normal deserialization. There is a lot of repeated work done for the same model over and over, so I added caching, and the performance is now roughly 1/10 of what it was. Ideally, there would be some way to specify the whole type mapping dictionary.

openapi-generator version

4.2.3

OpenAPI declaration file content or url

components:
  schemas:
    Animal:
      type: object
      required:
      - type_key
      properties:
        type_key:
          type: string
      mapping:
        pet: '#/components/schemas/Pet'
    Pet:
      type: object
      properties:
        type_key:
          type: string
      discriminator:
        propertyName: type_key
        mapping:
          cat: '#/components/schemas/Cat'
    Cat:
      type: object
      properties:
        type_key:
          age: number

Suggest a fix

See Description

[spacether]

Can you please submit your prosed PR from eak24@f606cfb
As a fix for this issue?

[eak24]

@spacether, #4913 is submitted as a PR - still nowhere close to final, it isn't performant for my use case, so I'd like some advice on how to improve that and whether or not this is inline with the OAS.