youknowone · Oct 27, 2026 · Oct 13, 2026
diff --git a/crates/stdlib/src/_asyncio.rs b/crates/stdlib/src/_asyncio.rs
@@ -965,7 +965,17 @@ pub(crate) mod _asyncio {
         future: PyRwLock<Option<PyObjectRef>>,
     }
 
-    #[pyclass(with(IterNext, Iterable))]
+    impl Constructor for PyFutureIter {
+        type Args = ();
+
+        fn py_new(_cls: &Py<PyType>, _args: Self::Args, _vm: &VirtualMachine) -> PyResult<Self> {
+            Ok(Self {
+                future: PyRwLock::new(None),
+            })
+        }
+    }
+
+    #[pyclass(with(Constructor, IterNext, Iterable))]
     impl PyFutureIter {
         #[pymethod]
         fn send(zelf: &Py<Self>, _value: PyObjectRef, vm: &VirtualMachine) -> PyResult {
@@ -2926,7 +2936,17 @@ pub(crate) mod _asyncio {
         task: PyRwLock<Option<PyObjectRef>>,
     }
 
-    #[pyclass(with(Callable, Representable))]
+    impl Constructor for TaskStepMethWrapper {
+        type Args = ();
+
+        fn py_new(_cls: &Py<PyType>, _args: Self::Args, _vm: &VirtualMachine) -> PyResult<Self> {
+            Ok(Self {
+                task: PyRwLock::new(None),
+            })
+        }
+    }
+
+    #[pyclass(with(Constructor, Callable, Representable))]
     impl TaskStepMethWrapper {
         fn new(task: PyObjectRef) -> Self {
             Self {

diff --git a/crates/vm/src/builtins/type.rs b/crates/vm/src/builtins/type.rs
@@ -1219,7 +1219,7 @@ impl PyType {
         // Note: inherit_slots is called in PyClassImpl::init_class after
         // slots are fully initialized by make_slots()
 
-        Self::set_new(&new_type.slots, new_type.base.deref());
+        Self::set_new(&new_type.slots, new_type.base.deref(), false);
         Self::set_alloc(&new_type.slots, new_type.base.deref());
 
         let weakref_type = super::PyWeak::static_type();
@@ -1271,7 +1271,7 @@ impl PyType {
             self.update_slot::<true>(attr_name, ctx);
         }
 
-        Self::set_new(&self.slots, self.base.deref());
+        Self::set_new(&self.slots, self.base.deref(), true);
         Self::set_alloc(&self.slots, self.base.deref());
     }
 
@@ -1295,11 +1295,38 @@ impl PyType {
         }
     }
 
-    fn set_new(slots: &PyTypeSlots, base: Option<&Py<Self>>) {
+    /// type_ready_set_new: a static type with no tp_new whose base is object
+    /// gets DISALLOW_INSTANTIATION and keeps tp_new unset. Heap types inherit
+    /// the base's tp_new. A static type that inherits a non-NULL tp_new must
+    /// have the same basicsize as that base, because the inherited function
+    /// allocates the base payload. Heap types, including native types flagged
+    /// HEAPTYPE, reuse the base payload like classes created at runtime.
+    fn set_new(slots: &PyTypeSlots, base: Option<&Py<Self>>, heap: bool) {
+        let is_heap = heap || slots.flags.has_feature(PyTypeFlags::HEAPTYPE);
+
+        if !is_heap
+            && slots.new.load().is_none()
+            && base.is_some_and(|b| core::ptr::eq(b, object::PyBaseObject::static_type()))
+        {
+            slots.flags.insert(PyTypeFlags::DISALLOW_INSTANTIATION);
+        }
+
         if slots.flags.has_feature(PyTypeFlags::DISALLOW_INSTANTIATION) {
-            slots.new.store(None)
+            slots.new.store(None);
         } else if slots.new.load().is_none() {
-            slots.new.store(base.and_then(|base| base.slots.new.load()))
+            let inherited = base.and_then(|base| base.slots.new.load());
+            if !is_heap
+                && inherited.is_some()
+                && let Some(base) = base
+                && slots.basicsize != base.slots.basicsize
+            {
+                panic!(
+                    "static type '{}' inherits tp_new from '{}' but basicsize {} != {}; \
+                     the inherited tp_new allocates the base payload",
+                    slots.name, base.slots.name, slots.basicsize, base.slots.basicsize
+                );
+            }
+            slots.new.store(inherited);
         }
     }
 
@@ -1312,7 +1339,7 @@ impl PyType {
     }
 
     pub(crate) fn finalize_bootstrap_static(typ: &Py<Self>) {
-        Self::set_new(&typ.slots, typ.base.deref());
+        Self::set_new(&typ.slots, typ.base.deref(), false);
         Self::set_alloc(&typ.slots, typ.base.deref());
     }
 

diff --git a/crates/vm/src/class.rs b/crates/vm/src/class.rs
@@ -367,8 +367,14 @@ pub trait PyClassImpl: PyClassDef {
 
         // Don't add __new__ attribute if slot_new is inherited from object
         // (Python doesn't add __new__ to __dict__ for inherited slots)
-        // Exception: object itself should have __new__ in its dict
-        if let Some(slot_new) = class.slots().new.load() {
+        // Exception: object itself should have __new__ in its dict.
+        // DISALLOW_INSTANTIATION types must not expose an inherited __new__.
+        if !class
+            .slots()
+            .flags
+            .has_feature(PyTypeFlags::DISALLOW_INSTANTIATION)
+            && let Some(slot_new) = class.slots().new.load()
+        {
             let object_new = ctx.types.object_type.slots().new.load();
             let is_object_itself = core::ptr::eq(class, ctx.types.object_type);
             let is_inherited_from_object = !is_object_itself

diff --git a/crates/vm/src/stdlib/_io.rs b/crates/vm/src/stdlib/_io.rs
@@ -520,7 +520,7 @@ mod _io {
 
     #[pyclass(
         with(IterNext, Iterable, Destructor),
-        flags(BASETYPE, HAS_DICT, HAS_WEAKREF)
+        flags(BASETYPE, HAS_DICT, HAS_WEAKREF, HEAPTYPE)
     )]
     impl _IOBase {
         #[pymethod]

diff --git a/crates/vm/src/stdlib/_testinternalcapi.rs b/crates/vm/src/stdlib/_testinternalcapi.rs
@@ -42,8 +42,8 @@ mod _testinternalcapi {
         protocol::{PyIterReturn, PyMappingMethods, PySequenceMethods},
         stdlib::time::pytime::{self, Round},
         types::{
-            AsMapping, AsSequence, Comparable, IterNext, Iterable, PyComparisonOp, PyTypeFlags,
-            SelfIter,
+            AsMapping, AsSequence, Comparable, Constructor, IterNext, Iterable, PyComparisonOp,
+            PyTypeFlags, SelfIter,
         },
     };
     #[cfg(any(unix, windows))]
@@ -779,7 +779,18 @@ mod _testinternalcapi {
         }
     }
 
-    #[pyclass(flags(HAS_WEAKREF), with(AsMapping, AsSequence, Comparable, Iterable))]
+    impl Constructor for Hamt {
+        type Args = ();
+
+        fn py_new(_cls: &Py<PyType>, _args: Self::Args, _vm: &VirtualMachine) -> PyResult<Self> {
+            Ok(Self::default())
+        }
+    }
+
+    #[pyclass(
+        flags(HAS_WEAKREF),
+        with(Constructor, AsMapping, AsSequence, Comparable, Iterable)
+    )]
     impl Py<Hamt> {
         #[pymethod]
         fn set(
@@ -1122,7 +1133,16 @@ mod _testinternalcapi {
     }
 
     #[cfg(feature = "codegen")]
-    #[pyclass(with(Py))]
+    impl Constructor for PyInstructionSequence {
+        type Args = ();
+
+        fn py_new(_cls: &Py<PyType>, _args: Self::Args, _vm: &VirtualMachine) -> PyResult<Self> {
+            Ok(Self::empty())
+        }
+    }
+
+    #[cfg(feature = "codegen")]
+    #[pyclass(with(Constructor, Py))]
     impl PyInstructionSequence {}
 
     #[cfg(feature = "codegen")]

diff --git a/crates/vm/src/stdlib/itertools.rs b/crates/vm/src/stdlib/itertools.rs
@@ -706,7 +706,30 @@ mod decl {
         tgtkey: PyObjectRef,
     }
 
-    #[pyclass(with(IterNext, Iterable), flags(HAS_WEAKREF))]
+    #[derive(FromArgs)]
+    struct GrouperArgs {
+        #[pyarg(positional)]
+        parent: PyRef<PyItertoolsGroupBy>,
+        #[pyarg(positional)]
+        tgtkey: PyObjectRef,
+    }
+
+    impl Constructor for PyItertoolsGrouper {
+        type Args = GrouperArgs;
+
+        fn py_new(
+            _cls: &Py<PyType>,
+            Self::Args { parent, tgtkey }: Self::Args,
+            _vm: &VirtualMachine,
+        ) -> PyResult<Self> {
+            Ok(Self {
+                groupby: parent,
+                tgtkey,
+            })
+        }
+    }
+
+    #[pyclass(with(Constructor, IterNext, Iterable), flags(HAS_WEAKREF))]
     impl PyItertoolsGrouper {}
 
     impl SelfIter for PyItertoolsGrouper {}