Add QuerySetAny as a non-generic variant of QuerySet.

This also re-export `QuerySetAny` for external access to
nongeneric QuerySet.

The approach taken here is making `_QuerySetAny` an alias
of `_QuerySet[_T, _T]` dedicated for isinstance checks, and
leave `QuerySet` unchanged as the type alias of `_QuerySet[_T, _T]`.

Fixes typeddjango#704.

Signed-off-by: Zixuan James Li <p359101898@gmail.com>

README.md

@@ -270,6 +270,30 @@ func(MyModel.objects.annotate(foo=Value("")).get(id=1))  # OK
 func(MyModel.objects.annotate(bar=Value("")).get(id=1))  # Error
 ```
 
+### How do I check if something is an instance of QuerySet in runtime?
+
+A limitation of making `QuerySet` generic is that you can not use
+it for `isinstance` checks.
+
+```python
+from django.db.models.query import QuerySet
+
+def foo(obj: object) -> None:
+    isinstance(obj, QuerySet) # Error: Parameterized generics cannot be used with class or instance checks
+```
+
+To get around with this issue without making `QuerySet` non-generic,
+Django-stubs provides `django_stubs_ext.QuerySetAny`, a non-generic
+variant of `QuerySet` dedicated for runtime type checking:
+
+```python
+from django_stubs_ext import QuerySetAny
+
+def foo(obj: object) -> None:
+    isinstance(obj, QuerySetAny)  # OK
+```
+
+
 ## Related projects
 
 - [`awesome-python-typing`](https://github.com/typeddjango/awesome-python-typing) - Awesome list of all typing-related things in Python.

django-stubs/db/models/query.pyi

@@ -209,6 +209,8 @@ class RawQuerySet(Iterable[_T], Sized):
     def resolve_model_init_order(self) -> Tuple[List[str], List[int], List[Tuple[str, int]]]: ...
     def using(self, alias: Optional[str]) -> RawQuerySet[_T]: ...
 
+_QuerySetAny = _QuerySet
+
 QuerySet = _QuerySet[_T, _T]
 
 class Prefetch:

django_stubs_ext/django_stubs_ext/__init__.py

@@ -1,3 +1,4 @@
+from .aliases import QuerySetAny as QuerySetAny
 from .aliases import StrOrPromise, StrPromise
 from .aliases import ValuesQuerySet as ValuesQuerySet
 from .annotations import Annotations as Annotations
@@ -7,6 +8,7 @@
 
 __all__ = [
     "monkeypatch",
+    "QuerySetAny",
     "ValuesQuerySet",
     "WithAnnotations",
     "Annotations",

django_stubs_ext/django_stubs_ext/aliases.py

@@ -1,16 +1,18 @@
 import typing
 
 if typing.TYPE_CHECKING:
-    from django.db.models.query import _T, _QuerySet, _Row
+    from django.db.models.query import _T, _QuerySet, _QuerySetAny, _Row
     from django.utils.functional import _StrOrPromise as StrOrPromise
     from django.utils.functional import _StrPromise as StrPromise
 
+    QuerySetAny = _QuerySetAny
     ValuesQuerySet = _QuerySet[_T, _Row]
 else:
     from django.db.models.query import QuerySet
     from django.utils.functional import Promise as StrPromise
 
+    QuerySetAny = QuerySet
     ValuesQuerySet = QuerySet
     StrOrPromise = typing.Union[str, StrPromise]
 
-__all__ = ["StrOrPromise", "StrPromise", "ValuesQuerySet"]
+__all__ = ["StrOrPromise", "StrPromise", "QuerySetAny", "ValuesQuerySet"]

tests/typecheck/managers/querysets/test_querysetalias.yml

@@ -0,0 +1,45 @@
+- case: queryset_isinstance_check
+  main: |
+    from typing import Any
+    from django.db.models.query import QuerySet
+    from django_stubs_ext import QuerySetAny
+
+    def foo(q: QuerySet[Any]) -> None:
+      pass
+
+    def bar(q: QuerySetAny) -> None:
+      pass
+
+    def baz(obj: object) -> None:
+      if isinstance(obj, QuerySetAny):
+        reveal_type(obj)             # N: Revealed type is "django.db.models.query._QuerySet[Any, Any]"
+        foo(obj)
+        bar(obj)
+
+      if isinstance(obj, QuerySet):  # E: Parameterized generics cannot be used with class or instance checks
+        reveal_type(obj)             # N: Revealed type is "django.db.models.query._QuerySet[Any, Any]"
+        foo(obj)
+        bar(obj)
+- case: queryset_list
+  main: |
+    from typing import List
+    from django.db.models.query import QuerySet
+    from django_stubs_ext import QuerySetAny
+    from myapp.models import User, Book
+
+    def try_append(queryset_instance: QuerySetAny, queryset: QuerySet[User], queryset_book: QuerySet[Book]) -> None:
+      user_querysets: List[QuerySet[User]] = []
+      user_querysets.append(queryset_instance)
+      user_querysets.append(queryset)
+      user_querysets.append(queryset_book)  # E: Argument 1 to "append" of "list" has incompatible type "_QuerySet[Book, Book]"; expected "_QuerySet[User, User]"
+  installed_apps:
+    - myapp
+  files:
+    - path: myapp/__init__.py
+    - path: myapp/models.py
+      content: |
+        from django.db import models
+        class User(models.Model):
+          pass
+        class Book(models.Model):
+          pass