ref: add mypy plugin to fix referencing .objects through a TypeVar (#72877)

fortunately there's enough plugin hooks that we can work around
python/mypy#17395

<!-- Describe your PR here. -->

Author: asottile-sentry

src/sentry/hybridcloud/models/webhookpayload.py

@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 import datetime
-from typing import Any
+from typing import Any, Self
 
 from django.db import models
 from django.http import HttpRequest
@@ -79,7 +79,7 @@ def create_from_request(
         identifier: int | str,
         request: HttpRequest,
         integration_id: int | None = None,
-    ) -> WebhookPayload:
+    ) -> Self:
         metrics.incr("hybridcloud.deliver_webhooks.saved")
         return cls.objects.create(
             mailbox_name=f"{provider}:{identifier}",

tests/tools/mypy_helpers/test_plugin.py

@@ -234,3 +234,78 @@ def _mypy() -> tuple[int, str]:
     cfg.write_text('[tool.mypy]\nplugins = ["tools.mypy_helpers.plugin"]\n')
     ret, out = _mypy()
     assert ret == 0
+
+
+def test_resolution_of_objects_across_typevar(tmp_path: pathlib.Path) -> None:
+    src = """\
+from typing import assert_type, TypeVar
+
+from sentry.db.models.base import Model
+
+M = TypeVar("M", bound=Model, covariant=True)
+
+def f(m: type[M]) -> M:
+    return m.objects.get()
+
+class C(Model): pass
+
+assert_type(f(C), C)
+"""
+    expected = """\
+<string>:8: error: Incompatible return value type (got "Model", expected "M")  [return-value]
+Found 1 error in 1 file (checked 1 source file)
+"""
+
+    # tools tests aren't allowed to import from `sentry` so we fixture
+    # the particular source file we are testing
+    models_dir = tmp_path.joinpath("sentry/db/models")
+    models_dir.mkdir(parents=True)
+
+    models_base_src = """\
+from typing import ClassVar, Self
+
+from .manager.base import BaseManager
+
+class Model:
+    objects: ClassVar[BaseManager[Self]]
+"""
+    models_dir.joinpath("base.pyi").write_text(models_base_src)
+
+    manager_dir = models_dir.joinpath("manager")
+    manager_dir.mkdir(parents=True)
+
+    manager_base_src = """\
+from typing import Generic, TypeVar
+
+M = TypeVar("M")
+
+class BaseManager(Generic[M]):
+    def get(self) -> M: ...
+    """
+    manager_dir.joinpath("base.pyi").write_text(manager_base_src)
+
+    cfg = tmp_path.joinpath("mypy.toml")
+    cfg.write_text("[tool.mypy]\nplugins = []\n")
+
+    # can't use our helper above because we're fixturing sentry src, so mimic it here
+    def _mypy() -> tuple[int, str]:
+        ret = subprocess.run(
+            (
+                *(sys.executable, "-m", "mypy"),
+                *("--config", cfg),
+                *("-c", src),
+            ),
+            env={**os.environ, "MYPYPATH": str(tmp_path)},
+            capture_output=True,
+            encoding="UTF-8",
+        )
+        assert not ret.stderr
+        return ret.returncode, ret.stdout
+
+    ret, out = _mypy()
+    assert ret
+    assert out == expected
+
+    cfg.write_text('[tool.mypy]\nplugins = ["tools.mypy_helpers.plugin"]\n')
+    ret, out = _mypy()
+    assert ret == 0

tools/mypy_helpers/plugin.py

@@ -17,6 +17,8 @@
     NoneType,
     Type,
     TypeOfAny,
+    TypeType,
+    TypeVarType,
     UnionType,
 )
 
@@ -114,6 +116,23 @@ def _lazy_service_wrapper_attribute(ctx: AttributeContext, *, attr: str) -> Type
         return member
 
 
+def _resolve_objects_for_typevars(ctx: AttributeContext) -> Type:
+    # XXX: hack around python/mypy#17395
+
+    # self: type[<TypeVar>]
+    # default_attr_type: BaseManager[ConcreteTypeVarBound]
+    if (
+        isinstance(ctx.type, TypeType)
+        and isinstance(ctx.type.item, TypeVarType)
+        and isinstance(ctx.default_attr_type, Instance)
+        and ctx.default_attr_type.type.fullname == "sentry.db.models.manager.base.BaseManager"
+    ):
+        tvar = ctx.type.item
+        return ctx.default_attr_type.copy_modified(args=(tvar,))
+    else:
+        return ctx.default_attr_type
+
+
 class SentryMypyPlugin(Plugin):
     def get_function_signature_hook(
         self, fullname: str
@@ -127,6 +146,12 @@ def get_base_class_hook(self, fullname: str) -> Callable[[ClassDefContext], None
         else:
             return None
 
+    def get_class_attribute_hook(self, fullname: str) -> Callable[[AttributeContext], Type] | None:
+        if fullname.startswith("sentry.") and fullname.endswith(".objects"):
+            return _resolve_objects_for_typevars
+        else:
+            return None
+
     def get_attribute_hook(self, fullname: str) -> Callable[[AttributeContext], Type] | None:
         if fullname.startswith("sentry.utils.lazy_service_wrapper.LazyServiceWrapper."):
             _, attr = fullname.rsplit(".", 1)