Classify __slots__ as class variables

mypy/plugins/attrs.py

@@ -960,7 +960,9 @@ def _add_slots(ctx: mypy.plugin.ClassDefContext, attributes: list[Attribute]) ->
         [ctx.api.named_type("builtins.str") for _ in attributes],
         fallback=ctx.api.named_type("builtins.tuple"),
     )
-    add_attribute_to_class(api=ctx.api, cls=ctx.cls, name="__slots__", typ=slots_type)
+    add_attribute_to_class(
+        api=ctx.api, cls=ctx.cls, name="__slots__", typ=slots_type, is_classvar=True
+    )
 
 
 def _add_match_args(ctx: mypy.plugin.ClassDefContext, attributes: list[Attribute]) -> None:

mypy/plugins/dataclasses.py

@@ -475,6 +475,7 @@ def add_slots(self, info: TypeInfo, attributes: list[DataclassAttribute]) -> Non
             self._cls,
             "__slots__",
             slots_type,
+            is_classvar=True,
             overwrite_existing=slots_defined_by_plugin,
         )
 

mypy/semanal.py

@@ -4648,6 +4648,7 @@ def make_name_lvalue_var(
             assert self.type is not None
             v.info = self.type
             v.is_initialized_in_class = True
+            v.is_classvar = name == "__slots__"
             v.allow_incompatible_override = name in ALLOW_INCOMPATIBLE_OVERRIDE
         if kind != LDEF:
             v._fullname = self.qualified_name(name)

mypyc/irbuild/classdef.py

@@ -329,6 +329,9 @@ def add_attr(self, lvalue: NameExpr, stmt: AssignmentStmt) -> None:
         # Variable declaration with no body
         if isinstance(stmt.rvalue, TempNode):
             return
+        # Native classes use mypyc's own layout, so don't emit __slots__ at runtime.
+        if lvalue.name == "__slots__":
+            return
         # Only treat marked class variables as class variables.
         if not (is_class_var(lvalue) or stmt.is_final_def):
             return

mypyc/test-data/run-classes.test

@@ -1175,6 +1175,23 @@ assert f() == 10
 A.x = 200
 assert f() == 200
 
+[case testSlotsNotEmittedAsClassVar]
+class Base:
+    __slots__ = ("value",)
+    value: int
+
+class Child(Base):
+    pass
+
+def make_child() -> Child:
+    child = Child()
+    child.value = 1
+    return child
+[file driver.py]
+from native import make_child
+
+assert make_child().value == 1
+
 [case testClassVarDoesNotShadowMethodGlobal]
 from typing import ClassVar
 from testutil import assertRaises

test-data/unit/check-dataclasses.test

@@ -1514,6 +1514,18 @@ class Some:
         self.y = 1  # E: Trying to assign name "y" that is not in "__slots__" of type "__main__.Some"
 [builtins fixtures/dataclasses.pyi]
 
+[case testDataclassGeneratedSlotsClassVariableOverride]
+from dataclasses import dataclass
+from typing import ClassVar
+
+@dataclass(slots=True)
+class Base:
+    value: int
+
+class Child(Base):
+    __slots__: ClassVar[tuple[str, ...]] = ()
+[builtins fixtures/tuple.pyi]
+
 [case testDataclassWithSlotsDef]
 from dataclasses import dataclass
 

test-data/unit/check-plugin-attrs.test

@@ -1745,6 +1745,19 @@ class C:
         self.c = 2  # E: Trying to assign name "c" that is not in "__slots__" of type "__main__.C"
 [builtins fixtures/plugin_attrs.pyi]
 
+[case testAttrsGeneratedSlotsClassVariableOverride]
+from typing import ClassVar
+
+import attrs
+
+@attrs.define(slots=True)
+class Base:
+    value: int
+
+class Child(Base):
+    __slots__: ClassVar[tuple[str, ...]] = ()
+[builtins fixtures/plugin_attrs.pyi]
+
 [case testAttrsClassWithSlotsDerivedFromNonSlots]
 import attrs
 

test-data/unit/check-slots.test

@@ -85,6 +85,17 @@ A.b = 1
 [builtins fixtures/tuple.pyi]
 
 
+[case testSlotsClassVariableOverride]
+from typing import ClassVar
+
+class Base:
+    __slots__ = ()
+
+class Child(Base):
+    __slots__: ClassVar[tuple[str, ...]] = ()
+[builtins fixtures/tuple.pyi]
+
+
 [case testSlotsDefinitionMultipleVars1]
 class A:
     __slots__ = __fields__ = ("a", "b")