make FormMixin generic to allow proper typing for LoginView (#515)

closes #514

django-stubs/contrib/auth/views.pyi

@@ -1,6 +1,7 @@
 from typing import Any, Optional, Set
 
 from django.contrib.auth.base_user import AbstractBaseUser
+from django.contrib.auth.forms import AuthenticationForm
 from django.core.handlers.wsgi import WSGIRequest
 from django.http.request import HttpRequest
 from django.http.response import HttpResponseRedirect
@@ -14,7 +15,7 @@ class SuccessURLAllowedHostsMixin:
     success_url_allowed_hosts: Any = ...
     def get_success_url_allowed_hosts(self) -> Set[str]: ...
 
-class LoginView(SuccessURLAllowedHostsMixin, FormView):
+class LoginView(SuccessURLAllowedHostsMixin, FormView[AuthenticationForm]):
     authentication_form: Any = ...
     redirect_field_name: Any = ...
     redirect_authenticated_user: bool = ...

django-stubs/views/generic/edit.pyi

@@ -1,4 +1,4 @@
-from typing import Any, Callable, Dict, Optional, Sequence, Type, Union
+from typing import Any, Callable, Dict, Generic, Optional, Sequence, Type, TypeVar, Union
 
 from django.forms.forms import BaseForm
 from django.forms.models import BaseModelForm
@@ -8,6 +8,8 @@ from typing_extensions import Literal
 
 from django.http import HttpRequest, HttpResponse
 
+_FormT = TypeVar('_FormT', bound=BaseForm)
+
 class AbstractFormMixin(ContextMixin):
     initial: Dict[str, Any] = ...
     form_class: Optional[Type[BaseForm]] = ...
@@ -18,11 +20,11 @@ class AbstractFormMixin(ContextMixin):
     def get_form_kwargs(self) -> Dict[str, Any]: ...
     def get_success_url(self) -> str: ...
 
-class FormMixin(AbstractFormMixin):
-    def get_form_class(self) -> Type[BaseForm]: ...
-    def get_form(self, form_class: Optional[Type[BaseForm]] = ...) -> BaseForm: ...
-    def form_valid(self, form: BaseForm) -> HttpResponse: ...
-    def form_invalid(self, form: BaseForm) -> HttpResponse: ...
+class FormMixin(Generic[_FormT], AbstractFormMixin):
+    def get_form_class(self) -> Type[_FormT]: ...
+    def get_form(self, form_class: Optional[Type[_FormT]] = ...) -> BaseForm: ...
+    def form_valid(self, form: _FormT) -> HttpResponse: ...
+    def form_invalid(self, form: _FormT) -> HttpResponse: ...
 
 class ModelFormMixin(AbstractFormMixin, SingleObjectMixin):
     fields: Optional[Union[Sequence[str], Literal["__all__"]]] = ...
@@ -36,8 +38,8 @@ class ProcessFormView(View):
     def post(self, request: HttpRequest, *args: str, **kwargs: Any) -> HttpResponse: ...
     def put(self, *args: str, **kwargs: Any) -> HttpResponse: ...
 
-class BaseFormView(FormMixin, ProcessFormView): ...
-class FormView(TemplateResponseMixin, BaseFormView): ...
+class BaseFormView(FormMixin[_FormT], ProcessFormView): ...
+class FormView(TemplateResponseMixin, BaseFormView[_FormT]): ...
 class BaseCreateView(ModelFormMixin, ProcessFormView): ...
 class CreateView(SingleObjectTemplateResponseMixin, BaseCreateView): ...
 class BaseUpdateView(ModelFormMixin, ProcessFormView): ...

tests/typecheck/test_views.yml

@@ -0,0 +1,12 @@
+-   case: login_form_form_valid_typechecks
+    main: |
+        from django.contrib.auth.views import LoginView
+        from django.contrib.auth import login as auth_login
+        from django.http import HttpResponseRedirect
+        from django.contrib.auth.forms import AuthenticationForm
+
+        class MyLoginView(LoginView):
+            def form_valid(self, form: AuthenticationForm) -> HttpResponseRedirect:
+                """Ensure that form can have type AuthenticationForm."""
+                form.get_user()
+                return HttpResponseRedirect(self.get_success_url())