OpenAPI schema generation does not handle generic views without serializer classes

[Lucidiot]

This issue occurs when using the updated OpenAPI schema generation by @carltongibson (#6532).

Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

Run ./manage.py generateschema on any project that has an API endpoint that inherits from GenericAPIView and does not have a serializer_class.

Expected behavior

A normal OpenAPI schema, except the request and response parameters should be empty? Maybe with a warning. I am not sure whether this should raise an exception or handle those cases.

Actual behavior

Traceback (most recent call last):
  File "./manage.py", line 22, in <module>
    execute_from_command_line(sys.argv)
  File "/home/lucidiot/.virtualenvs/backend/lib/python3.6/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
    utility.execute()
  File "/home/lucidiot/.virtualenvs/backend/lib/python3.6/site-packages/django/core/management/__init__.py", line 375, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/lucidiot/.virtualenvs/backend/lib/python3.6/site-packages/django/core/management/base.py", line 316, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/lucidiot/.virtualenvs/backend/lib/python3.6/site-packages/django/core/management/base.py", line 353, in execute
    output = self.handle(*args, **options)
  File "/home/lucidiot/Prog/django-rest-framework/rest_framework/management/commands/generateschema.py", line 24, in handle
    schema = generator.get_schema(request=None, public=True)
  File "/home/lucidiot/Prog/django-rest-framework/rest_framework/schemas/generators.py", line 519, in get_schema
    paths = self.get_paths(None if public else request)
  File "/home/lucidiot/Prog/django-rest-framework/rest_framework/schemas/generators.py", line 506, in get_paths
    operation = view.schema.get_operation(path, method)
  File "/home/lucidiot/Prog/django-rest-framework/rest_framework/schemas/inspectors.py", line 527, in get_operation
    operation['responses'] = self._get_responses(path, method)
  File "/home/lucidiot/Prog/django-rest-framework/rest_framework/schemas/inspectors.py", line 734, in _get_responses
    serializer = view.get_serializer()
  File "/home/lucidiot/Prog/django-rest-framework/rest_framework/generics.py", line 110, in get_serializer
    serializer_class = self.get_serializer_class()
  File "/home/lucidiot/Prog/django-rest-framework/rest_framework/generics.py", line 127, in get_serializer_class
    % self.__class__.__name__
AssertionError: 'SomeAPIView' should either include a `serializer_class` attribute, or override the `get_serializer_class()` method.

[carltongibson]

Hey @tomchristie. Can I get you're opinion here?

We're catching an APIException here, like this:

try:
    serializer = view.get_serializer()
except exceptions.APIException:
    serializer = None
    warnings.warn('{}.get_serializer() raised an exception during '
                  'schema generation. Serializer fields will not be '
                  'generated for {} {}.'
                  .format(view.__class__.__name__, method, path))

We could also catch the AssertionError but having this raise has been standard policy for as long as I can remember, so I'm inclined to just say, "Yep wontfix: define serializer_class like it says. (Or drop down to APIView instead.)"

What do you reckon?
Ta.

[tomchristie]

so I'm inclined to just say, "Yep wontfix: define serializer_class like it says. (Or drop down to APIView instead.

Exactly yes. Or generate the schema and then edit part of it manually. Or whatever.

[carltongibson]

Fine. Thanks Tom.

Essentially, serializer_class, or overriding get_serializer() is a requirement for GenericAPIView. If you don't want that stick to APIView.

I hope that makes sense @Lucidiot? Thanks for the report and info!

[Lucidiot]

Okay, that makes sense. I was not getting this error anywhere else because the class was written like this:

class SomeAPIView(RetrieveAPIView):
    def retrieve(self, request, *args, **kwargs):
        # ...
        return Response({'url': '...'})

Obviously, since the get_serializer or get_serializer_class methods were never called, the error never occured.