Fix incorrect instance arg for BaseInlineFormSet.__init__ (#928)

* Fix BaseInlineFormSet generic to retain parent model class info

* Add in test for generic formset improvements

django-stubs/contrib/admin/options.pyi

@@ -296,7 +296,7 @@ _ParentModelT = TypeVar("_ParentModelT", bound=Model)
 class InlineModelAdmin(Generic[_ChildModelT, _ParentModelT], BaseModelAdmin[_ChildModelT]):
     model: Type[_ChildModelT] = ...
     fk_name: Optional[str] = ...
-    formset: Type[BaseInlineFormSet[_ChildModelT, forms.ModelForm[_ChildModelT]]] = ...
+    formset: Type[BaseInlineFormSet[_ChildModelT, _ParentModelT, forms.ModelForm[_ChildModelT]]] = ...
     extra: int = ...
     min_num: Optional[int] = ...
     max_num: Optional[int] = ...
@@ -318,7 +318,7 @@ class InlineModelAdmin(Generic[_ChildModelT, _ParentModelT], BaseModelAdmin[_Chi
     def get_max_num(self, request: HttpRequest, obj: Optional[_ParentModelT] = ..., **kwargs: Any) -> Optional[int]: ...
     def get_formset(
         self, request: HttpRequest, obj: Optional[_ParentModelT] = ..., **kwargs: Any
-    ) -> Type[BaseInlineFormSet[_ChildModelT, forms.ModelForm[_ChildModelT]]]: ...
+    ) -> Type[BaseInlineFormSet[_ChildModelT, _ParentModelT, forms.ModelForm[_ChildModelT]]]: ...
     def get_queryset(self, request: HttpRequest) -> QuerySet[_ChildModelT]: ...
     def has_add_permission(self, request: HttpRequest, obj: Optional[_ParentModelT]) -> bool: ...  # type: ignore
     def has_change_permission(self, request: HttpRequest, obj: Optional[_ParentModelT] = ...) -> bool: ...  # type: ignore

django-stubs/forms/models.pyi

@@ -50,6 +50,7 @@ _ErrorMessages = Dict[str, Dict[str, str]]
 _FormFieldCallback = Callable[[models.Field], Field]
 
 _M = TypeVar("_M", bound=Model)
+_ParentM = TypeVar("_ParentM", bound=Model)
 
 def construct_instance(
     form: BaseForm, instance: _M, fields: Optional[Container[str]] = ..., exclude: Optional[Container[str]] = ...
@@ -187,8 +188,8 @@ def modelformset_factory(
     can_delete_extra: bool = ...,
 ) -> Type[BaseModelFormSet[_M, _ModelFormT]]: ...
 
-class BaseInlineFormSet(BaseModelFormSet[_M, _ModelFormT]):
-    instance: Model = ...
+class BaseInlineFormSet(Generic[_M, _ParentM, _ModelFormT], BaseModelFormSet[_M, _ModelFormT]):
+    instance: _ParentM
     save_as_new: bool = ...
     unique_fields: Collection[str] = ...
     fk: ForeignKey  # set by inlineformset_set
@@ -196,7 +197,7 @@ class BaseInlineFormSet(BaseModelFormSet[_M, _ModelFormT]):
         self,
         data: Optional[_DataT] = ...,
         files: Optional[_FilesT] = ...,
-        instance: Optional[_M] = ...,
+        instance: Optional[_ParentM] = ...,
         save_as_new: bool = ...,
         prefix: Optional[str] = ...,
         queryset: Optional[QuerySet[_M]] = ...,
@@ -210,7 +211,7 @@ class BaseInlineFormSet(BaseModelFormSet[_M, _ModelFormT]):
     def get_unique_error_message(self, unique_check: Sequence[str]) -> str: ...
 
 def inlineformset_factory(
-    parent_model: Type[Model],
+    parent_model: Type[_ParentM],
     model: Type[_M],
     form: Type[_ModelFormT] = ...,
     formset: Type[BaseInlineFormSet] = ...,
@@ -233,7 +234,7 @@ def inlineformset_factory(
     field_classes: Optional[Mapping[str, Type[Field]]] = ...,
     absolute_max: Optional[int] = ...,
     can_delete_extra: bool = ...,
-) -> Type[BaseInlineFormSet[_M, _ModelFormT]]: ...
+) -> Type[BaseInlineFormSet[_M, _ParentM, _ModelFormT]]: ...
 
 class InlineForeignKeyField(Field):
     disabled: bool

tests/typecheck/test_formsets.yml

@@ -0,0 +1,21 @@
+- case: inlineformset_factory
+  main: |
+    from typing import Any, Type
+    from django import forms
+    from myapp.models import Article, Category
+    ArticleFS: Type[forms.BaseInlineFormSet[Article, Category, Any]] = forms.inlineformset_factory(Category, Article)
+    ArticleFS(instance=Article())  # E: Argument "instance" to "BaseInlineFormSet" has incompatible type "Article"; expected "Optional[Category]"
+    fs = ArticleFS(instance=Category())
+    reveal_type(fs.instance)  # N: Revealed type is "myapp.models.Category*"
+  installed_apps:
+    - myapp
+  files:
+    - path: myapp/__init__.py
+    - path: myapp/models.py
+      content: |
+        from django.db import models
+
+        class Article(models.Model):
+            pass
+        class Category(models.Model):
+            pass