It would be helpful to allow oauthlib.oauth2.WebApplicationClient to support supplementary response_types in addition to code. For example, for Azure a response_type=id_token+code returns an additional id_token in the Access Token response, this contains useful information such as the email address of the user. See this Stackoverflow answer.

I was thinking the signature could be changed to prepare_request_uri(uri, redirect_uri=None, scope=None, state=None, response_type=None, **kwargs).

The if response_type is not None it could be added e.g.

_response_type = if response_type then 'code+' + response_type else 'code'
return prepare_grant_uri(uri, self.client_id, _response_type,
redirect_uri=redirect_uri, scope=scope, state=state, **kwargs)