[BUG] python-fastapi: models break on forward type references

Created by: chludwig-haufe

Description

I gave the (beta) python-fastapi generator a try on a PoC API. The API's response model contains a recursive data structure (some irrelevant attributes removed):

definitions:
  # ...
  SearchCategory:
    properties:
      query:
        type: string
      subcategories:
        items:
          $ref: '#/definitions/SearchCategory'
        type: array

The generator python-fastapi translates this into the following Python module :

# coding: utf-8

from datetime import date, datetime  # noqa: F401

import re  # noqa: F401
from typing import Any, Dict, List, Optional  # noqa: F401

from pydantic import AnyUrl, BaseModel, EmailStr, validator  # noqa: F401


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

    Do not edit the class manually.

    SearchCategory - a model defined in OpenAPI

        query: The query of this SearchCategory [Optional].
        subcategories: The subcategories of this SearchCategory [Optional].
    """

    query: Optional[str] = None
    subcategories: Optional[List[SearchCategory]] = None

This module fails to import because of the forwarding type reference subcategories: Optional[List[SearchCategory]]. This will be supported out of the box from Python 3.10 on only. Since Python 3.7, you can opt-in using a __future__ import. Otherwise, the forward reference needs to be given as a string (i.e., subcategories: Optional[List["SearchCategory"]].

openapi-generator version

Head of master branch as of June 8, 2021 (c379f5bc)

Suggest a fix

Add from __future__ import annotations to python-fastapi/model.mustache. This solves the issue at least for >= Python 3.7.

Figuring out in the template whether it's a forwarding reference and conditionally wrapping the reference into a string seems much harder.