RustPython · youknowone · Dec 28, 2025 · Dec 28, 2025 · Jul 21, 2025 · Dec 28, 2025
diff --git a/Lib/test/test_types.py b/Lib/test/test_types.py
@@ -591,8 +591,6 @@ def test_format_spec_errors(self):
         for code in 'xXobns':
             self.assertRaises(ValueError, format, 0, ',' + code)
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_internal_sizes(self):
         self.assertGreater(object.__basicsize__, 0)
         self.assertGreater(tuple.__itemsize__, 0)

diff --git a/crates/derive-impl/src/pyclass.rs b/crates/derive-impl/src/pyclass.rs
@@ -455,6 +455,7 @@ fn generate_class_def(
     });
     // If repr(transparent) with a base, the type has the same memory layout as base,
     // so basicsize should be 0 (no additional space beyond the base type)
+    // Otherwise, basicsize = sizeof(payload). The header size is added in __basicsize__ getter.
     let basicsize = if is_repr_transparent && base.is_some() {
         quote!(0)
     } else {

diff --git a/crates/vm/src/builtins/type.rs b/crates/vm/src/builtins/type.rs
@@ -782,8 +782,8 @@ impl PyType {
     }
 
     #[pygetset]
-    const fn __basicsize__(&self) -> usize {
-        self.slots.basicsize
+    fn __basicsize__(&self) -> usize {
+        crate::object::SIZEOF_PYOBJECT_HEAD + self.slots.basicsize
     }
 
     #[pygetset]
@@ -1338,17 +1338,24 @@ impl Constructor for PyType {
         let member_count: usize = base_member_count + heaptype_member_count;
 
         let mut flags = PyTypeFlags::heap_type_flags();
+
+        // Check if we may add dict
+        // We can only add a dict if the primary base class doesn't already have one
+        // In CPython, this checks tp_dictoffset == 0
+        let may_add_dict = !base.slots.flags.has_feature(PyTypeFlags::HAS_DICT);
+
         // Add HAS_DICT and MANAGED_DICT if:
-        // 1. __slots__ is not defined, OR
+        // 1. __slots__ is not defined AND base doesn't have dict, OR
         // 2. __dict__ is in __slots__
-        if heaptype_slots.is_none() || add_dict {
+        if (heaptype_slots.is_none() && may_add_dict) || add_dict {
             flags |= PyTypeFlags::HAS_DICT | PyTypeFlags::MANAGED_DICT;
         }
 
         let (slots, heaptype_ext) = {
             let slots = PyTypeSlots {
                 flags,
                 member_count,
+                itemsize: base.slots.itemsize,
                 ..PyTypeSlots::heap_default()
             };
             let heaptype_ext = HeapTypeExt {
@@ -2009,23 +2016,19 @@ fn calculate_meta_class(
     Ok(winner)
 }
 
+/// Returns true if the two types have different instance layouts.
+fn shape_differs(t1: &Py<PyType>, t2: &Py<PyType>) -> bool {
+    t1.__basicsize__() != t2.__basicsize__() || t1.slots.itemsize != t2.slots.itemsize
+}
+
 fn solid_base<'a>(typ: &'a Py<PyType>, vm: &VirtualMachine) -> &'a Py<PyType> {
     let base = if let Some(base) = &typ.base {
         solid_base(base, vm)
     } else {
         vm.ctx.types.object_type
     };
 
-    // Check for extra instance variables (CPython's extra_ivars)
-    let t_size = typ.__basicsize__();
-    let b_size = base.__basicsize__();
-    let t_itemsize = typ.slots.itemsize;
-    let b_itemsize = base.slots.itemsize;
-
-    // Has extra ivars if: sizes differ AND (has items OR t_size > b_size)
-    let has_extra_ivars = t_size != b_size && (t_itemsize > 0 || b_itemsize > 0 || t_size > b_size);
-
-    if has_extra_ivars { typ } else { base }
+    if shape_differs(typ, base) { typ } else { base }
 }
 
 fn best_base<'a>(bases: &'a [PyTypeRef], vm: &VirtualMachine) -> PyResult<&'a Py<PyType>> {

diff --git a/crates/vm/src/object/core.rs b/crates/vm/src/object/core.rs
@@ -114,6 +114,7 @@ pub(super) struct PyInner<T> {
 
     pub(super) payload: T,
 }
+pub(crate) const SIZEOF_PYOBJECT_HEAD: usize = std::mem::size_of::<PyInner<()>>();
 
 impl<T: fmt::Debug> fmt::Debug for PyInner<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {

diff --git a/crates/vm/src/object/mod.rs b/crates/vm/src/object/mod.rs
@@ -7,4 +7,5 @@ mod traverse_object;
 pub use self::core::*;
 pub use self::ext::*;
 pub use self::payload::*;
+pub(crate) use core::SIZEOF_PYOBJECT_HEAD;
 pub use traverse::{MaybeTraverse, Traverse, TraverseFn};