[BUG] python-fastapi: free-form objects cause a pydantic failure

Created by: chludwig-haufe

Description

I gave the (beta) python-fastapi generator a try on a PoC API. The API's response model contains some free-form object attributes like the following:

definition:
  # ...
  SearchSortCriteria:
    type: object

  SearchResultPaging:
    type: object

  SearchQuery:
    properties:
      searchTerm:
        type: string
      page:
        $ref: '#/definitions/SearchResultPaging'
      sorting:
        $ref: '#/definitions/SearchSortCriteria'

(Obviously, the API definition needs some more work, but that's besides the point.)

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 Dict, List, Optional  # noqa: F401

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

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

    Do not edit the class manually.

    SearchQuery - a model defined in OpenAPI

        search_term: The search_term of this SearchQuery [Optional].
        page: The page of this SearchQuery [Optional].
        sorting: The sorting of this SearchQuery [Optional].
    """

    search_term: Optional[str] = None
    page: Optional[object] = None
    sorting: Optional[object] = None

This module fails to import with an error as follows (stack trace shortened):

Traceback (most recent call last):
  File "/Users/ludwigc/.pyenv/versions/search-middleware/bin/uvicorn", line 8, in <module>
    sys.exit(main())
  File "/Users/ludwigc/.pyenv/versions/3.9.5/envs/search-middleware/lib/python3.9/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
[...]
  File "/Users/ludwigc/.pyenv/versions/3.9.5/lib/python3.9/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
[...]
  File "/Users/ludwigc/search-middleware/src/aurora/middleware/search/models/search_query.py", line 13, in <module>
    class SearchQuery(BaseModel):
  File "pydantic/main.py", line 299, in pydantic.main.ModelMetaclass.__new__
  File "pydantic/fields.py", line 411, in pydantic.fields.ModelField.infer
  File "pydantic/fields.py", line 342, in pydantic.fields.ModelField.__init__
  File "pydantic/fields.py", line 456, in pydantic.fields.ModelField.prepare
  File "pydantic/fields.py", line 670, in pydantic.fields.ModelField.populate_validators
  File "pydantic/validators.py", line 715, in find_validators
RuntimeError: no validator found for <class 'object'>, see `arbitrary_types_allowed` in Config

openapi-generator version

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

Suggest a fix

Adding a Pydantic Config class to each model class that allows arbitrary types does not feel right. That's an escape hatch that should be used only where indeed needed.

IMHO, the “correct” type hint for such free-form objects is Dict[str, Any].