diff --git a/Lib/test/test_codecs.py b/Lib/test/test_codecs.py
index 0d3d8c9e2d..736022599e 100644
--- a/Lib/test/test_codecs.py
+++ b/Lib/test/test_codecs.py
@@ -3189,7 +3189,6 @@ def test_readline(self):
                 sout = reader.readline()
                 self.assertEqual(sout, b"\x80")
 
-    @unittest.expectedFailure # TODO: RUSTPYTHON
     def test_buffer_api_usage(self):
         # We check all the transform codecs accept memoryview input
         # for encoding and decoding
diff --git a/Lib/test/test_memoryio.py b/Lib/test/test_memoryio.py
index 61d9b180e2..07d9d38d6e 100644
--- a/Lib/test/test_memoryio.py
+++ b/Lib/test/test_memoryio.py
@@ -724,8 +724,6 @@ class CBytesIOTest(PyBytesIOTest):
     ioclass = io.BytesIO
     UnsupportedOperation = io.UnsupportedOperation
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_bytes_array(self):
         super().test_bytes_array()
 
@@ -739,8 +737,6 @@ def test_flags(self):
     def test_getbuffer(self):
         super().test_getbuffer()
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_init(self):
         super().test_init()
 
@@ -770,8 +766,6 @@ def test_relative_seek(self):
     def test_seek(self):
         super().test_seek()
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_subclassing(self):
         super().test_subclassing()
 
@@ -884,8 +878,6 @@ def test_detach(self):
     def test_flags(self):
         super().test_flags()
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_init(self):
         super().test_init()
 
diff --git a/Lib/test/test_pickle.py b/Lib/test/test_pickle.py
index 070e277c2a..ea51b9d091 100644
--- a/Lib/test/test_pickle.py
+++ b/Lib/test/test_pickle.py
@@ -180,11 +180,6 @@ def test_oob_buffers(self): # TODO(RUSTPYTHON): Remove this test when it passes
     def test_oob_buffers_writable_to_readonly(self): # TODO(RUSTPYTHON): Remove this test when it passes
         return super().test_oob_buffers_writable_to_readonly()
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
-    def test_optional_frames(self): # TODO(RUSTPYTHON): Remove this test when it passes
-        return super().test_optional_frames()
-
     # TODO: RUSTPYTHON
     @unittest.expectedFailure
     def test_buffers_error(self): # TODO(RUSTPYTHON): Remove this test when it passes
@@ -220,11 +215,6 @@ def test_oob_buffers(self): # TODO(RUSTPYTHON): Remove this test when it passes
     def test_oob_buffers_writable_to_readonly(self): # TODO(RUSTPYTHON): Remove this test when it passes
         return super().test_oob_buffers_writable_to_readonly()
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
-    def test_optional_frames(self): # TODO(RUSTPYTHON): Remove this test when it passes
-        return super().test_optional_frames()
-
 
 class InMemoryPickleTests(AbstractPickleTests, AbstractUnpickleTests,
                           BigmemPickleTests, unittest.TestCase):
@@ -309,11 +299,6 @@ def test_in_band_buffers(self): # TODO(RUSTPYTHON): Remove this test when it pas
     def test_oob_buffers(self): # TODO(RUSTPYTHON): Remove this test when it passes
         return super().test_oob_buffers()
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
-    def test_optional_frames(self): # TODO(RUSTPYTHON): Remove this test when it passes
-        return super().test_optional_frames()
-
 class PersistentPicklerUnpicklerMixin(object):
 
     def dumps(self, arg, proto=None):
diff --git a/Lib/test/test_pickletools.py b/Lib/test/test_pickletools.py
index 6c38bef3d3..492f57cce2 100644
--- a/Lib/test/test_pickletools.py
+++ b/Lib/test/test_pickletools.py
@@ -97,11 +97,6 @@ def test_oob_buffers(self): # TODO(RUSTPYTHON): Remove this test when it passes
     def test_oob_buffers_writable_to_readonly(self): # TODO(RUSTPYTHON): Remove this test when it passes
         return super().test_oob_buffers_writable_to_readonly()
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
-    def test_optional_frames(self): # TODO(RUSTPYTHON): Remove this test when it passes
-        return super().test_optional_frames()
-
     # TODO: RUSTPYTHON
     @unittest.expectedFailure
     def test_py_methods(self): # TODO(RUSTPYTHON): Remove this test when it passes
diff --git a/Lib/test/test_urllib2.py b/Lib/test/test_urllib2.py
index 5a02b5db8e..399c94213a 100644
--- a/Lib/test/test_urllib2.py
+++ b/Lib/test/test_urllib2.py
@@ -641,8 +641,6 @@ def test_raise(self):
         self.assertRaises(urllib.error.URLError, o.open, req)
         self.assertEqual(o.calls, [(handlers[0], "http_open", (req,), {})])
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_http_error(self):
         # XXX http_error_default
         # http errors are a special case
@@ -666,8 +664,6 @@ def test_http_error(self):
             self.assertEqual((handler, method_name), got[:2])
             self.assertEqual(args, got[2])
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_processors(self):
         # *_request / *_response methods get called appropriately
         o = OpenerDirector()
@@ -874,8 +870,6 @@ def test_file(self):
                 self.assertEqual(req.type, "ftp")
             self.assertEqual(req.type == "ftp", ftp)
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_http(self):
 
         h = urllib.request.AbstractHTTPHandler()
@@ -1136,8 +1130,6 @@ def test_fixpath_in_weirdurls(self):
         self.assertEqual(newreq.host, 'www.python.org')
         self.assertEqual(newreq.selector, '')
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_errors(self):
         h = urllib.request.HTTPErrorProcessor()
         o = h.parent = MockOpener()
@@ -1163,8 +1155,6 @@ def test_errors(self):
         self.assertEqual(o.proto, "http")  # o.error called
         self.assertEqual(o.args, (req, r, 502, "Bad gateway", {}))
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_cookies(self):
         cj = MockCookieJar()
         h = urllib.request.HTTPCookieProcessor(cj)
@@ -1291,8 +1281,6 @@ def test_relative_redirect(self):
             MockHeaders({"location": valid_url}))
         self.assertEqual(o.req.get_full_url(), valid_url)
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_cookie_redirect(self):
         # cookies shouldn't leak into redirected requests
         from http.cookiejar import CookieJar
@@ -1308,8 +1296,6 @@ def test_cookie_redirect(self):
         o.open("http://www.example.com/")
         self.assertFalse(hh.req.has_header("Cookie"))
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_redirect_fragment(self):
         redirected_url = 'http://www.example.com/index.html#OK\r\n\r\n'
         hh = MockHTTPHandler(302, 'Location: ' + redirected_url)
@@ -1374,8 +1360,6 @@ def http_open(self, req):
                 request = handler.last_buf
                 self.assertTrue(request.startswith(expected), repr(request))
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_proxy(self):
         u = "proxy.example.com:3128"
         for d in dict(http=u), dict(HTTP=u):
@@ -1420,8 +1404,6 @@ def test_proxy_no_proxy_all(self):
         self.assertEqual(req.host, "www.python.org")
         del os.environ['no_proxy']
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_proxy_https(self):
         o = OpenerDirector()
         ph = urllib.request.ProxyHandler(dict(https="proxy.example.com:3128"))
@@ -1509,8 +1491,6 @@ def check_basic_auth(self, headers, realm):
                                   "http://acme.example.com/protected",
                                   "http://acme.example.com/protected")
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_basic_auth(self):
         realm = "realm2@example.com"
         realm2 = "realm2@example.com"
@@ -1556,8 +1536,6 @@ def test_basic_auth(self):
                        for challenge in challenges]
             self.check_basic_auth(headers, realm)
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_proxy_basic_auth(self):
         opener = OpenerDirector()
         ph = urllib.request.ProxyHandler(dict(http="proxy.example.com:3128"))
@@ -1575,8 +1553,6 @@ def test_proxy_basic_auth(self):
                               "proxy.example.com:3128",
                               )
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_basic_and_digest_auth_handlers(self):
         # HTTPDigestAuthHandler raised an exception if it couldn't handle a 40*
         # response (http://python.org/sf/1479302), where it should instead
@@ -1684,8 +1660,6 @@ def _test_basic_auth(self, opener, auth_handler, auth_header,
         self.assertEqual(len(http_handler.requests), 1)
         self.assertFalse(http_handler.requests[0].has_header(auth_header))
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_basic_prior_auth_auto_send(self):
         # Assume already authenticated if is_authenticated=True
         # for APIs like Github that don't return 401
@@ -1713,8 +1687,6 @@ def test_basic_prior_auth_auto_send(self):
         # expect request to be sent with auth header
         self.assertTrue(http_handler.has_auth_header)
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_basic_prior_auth_send_after_first_success(self):
         # Auto send auth header after authentication is successful once
 
diff --git a/Lib/test/test_zipfile.py b/Lib/test/test_zipfile.py
index 0a50f03604..e7705590cd 100644
--- a/Lib/test/test_zipfile.py
+++ b/Lib/test/test_zipfile.py
@@ -1746,8 +1746,6 @@ def test_empty_file_raises_BadZipFile(self):
             fp.write("short file")
         self.assertRaises(zipfile.BadZipFile, zipfile.ZipFile, TESTFN)
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_negative_central_directory_offset_raises_BadZipFile(self):
         # Zip file containing an empty EOCD record
         buffer = bytearray(b'PK\x05\x06' + b'\0'*18)
diff --git a/crates/derive-impl/src/pyclass.rs b/crates/derive-impl/src/pyclass.rs
index d1fe539863..1a01721391 100644
--- a/crates/derive-impl/src/pyclass.rs
+++ b/crates/derive-impl/src/pyclass.rs
@@ -223,8 +223,8 @@ pub(crate) fn impl_pyclass_impl(attr: PunctuatedNestedMeta, item: Item) -> Resul
                         const METHOD_DEFS: &'static [::rustpython_vm::function::PyMethodDef] = &#method_defs;
 
                         fn extend_slots(slots: &mut ::rustpython_vm::types::PyTypeSlots) {
-                            #impl_ty::__extend_slots(slots);
                             #with_slots
+                            #impl_ty::__extend_slots(slots);
                         }
                     }
                 }
@@ -1672,9 +1672,24 @@ fn extract_impl_attrs(attr: PunctuatedNestedMeta, item: &Ident) -> Result<Extrac
                             #extend_class(ctx, class);
                         });
                         with_method_defs.push(method_defs);
-                        with_slots.push(quote_spanned! { item_span =>
-                            #extend_slots(slots);
-                        });
+                        // For Initializer and Constructor traits, directly set the slot
+                        // instead of calling __extend_slots. This ensures that the trait
+                        // impl's override (e.g., slot_init in impl Initializer) is used,
+                        // not the trait's default implementation.
+                        let slot_code = if path.is_ident("Initializer") {
+                            quote_spanned! { item_span =>
+                                slots.init.store(Some(<Self as ::rustpython_vm::types::Initializer>::slot_init as _));
+                            }
+                        } else if path.is_ident("Constructor") {
+                            quote_spanned! { item_span =>
+                                slots.new.store(Some(<Self as ::rustpython_vm::types::Constructor>::slot_new as _));
+                            }
+                        } else {
+                            quote_spanned! { item_span =>
+                                #extend_slots(slots);
+                            }
+                        };
+                        with_slots.push(slot_code);
                     }
                 } else if path.is_ident("flags") {
                     for meta in nested {
diff --git a/crates/vm/src/builtins/object.rs b/crates/vm/src/builtins/object.rs
index ca208790f4..77a1eff6c7 100644
--- a/crates/vm/src/builtins/object.rs
+++ b/crates/vm/src/builtins/object.rs
@@ -120,44 +120,36 @@ impl Initializer for PyBaseObject {
 
     // object_init: excess_args validation
     fn slot_init(zelf: PyObjectRef, args: FuncArgs, vm: &VirtualMachine) -> PyResult<()> {
+        if args.is_empty() {
+            return Ok(());
+        }
+
         let typ = zelf.class();
         let object_type = &vm.ctx.types.object_type;
 
         let typ_init = typ.slots.init.load().map(|f| f as usize);
         let object_init = object_type.slots.init.load().map(|f| f as usize);
-        let typ_new = typ.slots.new.load().map(|f| f as usize);
-        let object_new = object_type.slots.new.load().map(|f| f as usize);
 
-        // For heap types (Python classes), check if __new__ is defined anywhere in MRO
-        // (before object) because heap types always have slots.new = new_wrapper via MRO
-        let is_heap_type = typ
-            .slots
-            .flags
-            .contains(crate::types::PyTypeFlags::HEAPTYPE);
-        let new_overridden = if is_heap_type {
-            // Check if __new__ is defined in any base class (excluding object)
-            let new_id = identifier!(vm, __new__);
-            typ.mro_collect()
-                .into_iter()
-                .take_while(|t| !std::ptr::eq(t.as_ref(), *object_type))
-                .any(|t| t.attributes.read().contains_key(new_id))
-        } else {
-            // For built-in types, use slot comparison
-            typ_new != object_new
-        };
-
-        // If both __init__ and __new__ are overridden, allow excess args
-        if typ_init != object_init && new_overridden {
-            return Ok(());
+        // if (type->tp_init != object_init) → first error
+        if typ_init != object_init {
+            return Err(vm.new_type_error(
+                "object.__init__() takes exactly one argument (the instance to initialize)"
+                    .to_owned(),
+            ));
         }
 
-        // Otherwise, reject excess args
-        if !args.is_empty() {
+        let typ_new = typ.slots.new.load().map(|f| f as usize);
+        let object_new = object_type.slots.new.load().map(|f| f as usize);
+
+        // if (type->tp_new == object_new) → second error
+        if typ_new == object_new {
             return Err(vm.new_type_error(format!(
                 "{}.__init__() takes exactly one argument (the instance to initialize)",
                 typ.name()
             )));
         }
+
+        // Both conditions false → OK (e.g., tuple, dict with custom __new__)
         Ok(())
     }
 
@@ -591,6 +583,13 @@ pub fn object_set_dict(obj: PyObjectRef, dict: PyDictRef, vm: &VirtualMachine) -
 }
 
 pub fn init(ctx: &Context) {
+    // Manually set init slot - derive macro doesn't generate extend_slots
+    // for trait impl that overrides #[pyslot] method
+    ctx.types
+        .object_type
+        .slots
+        .init
+        .store(Some(<PyBaseObject as Initializer>::slot_init));
     PyBaseObject::extend_class(ctx, ctx.types.object_type);
 }
 
diff --git a/crates/vm/src/builtins/type.rs b/crates/vm/src/builtins/type.rs
index ffaad32468..4f150d9258 100644
--- a/crates/vm/src/builtins/type.rs
+++ b/crates/vm/src/builtins/type.rs
@@ -25,8 +25,8 @@ use crate::{
     object::{Traverse, TraverseFn},
     protocol::{PyIterReturn, PyNumberMethods},
     types::{
-        AsNumber, Callable, Constructor, GetAttr, PyTypeFlags, PyTypeSlots, Representable, SetAttr,
-        TypeDataRef, TypeDataRefMut, TypeDataSlot,
+        AsNumber, Callable, Constructor, GetAttr, Initializer, PyTypeFlags, PyTypeSlots,
+        Representable, SetAttr, TypeDataRef, TypeDataRefMut, TypeDataSlot,
     },
 };
 use indexmap::{IndexMap, map::Entry};
@@ -412,8 +412,10 @@ impl PyType {
     }
 
     pub(crate) fn init_slots(&self, ctx: &Context) {
-        // Inherit slots from direct bases (not MRO)
-        for base in self.bases.read().iter() {
+        // Inherit slots from MRO
+        // Note: self.mro does NOT include self, so we iterate all elements
+        let mro: Vec<_> = self.mro.read().iter().cloned().collect();
+        for base in mro.iter() {
             self.inherit_slots(base);
         }
 
@@ -460,7 +462,7 @@ impl PyType {
         }
     }
 
-    /// Inherit slots from base type. typeobject.c: inherit_slots
+    /// Inherit slots from base type. inherit_slots
     pub(crate) fn inherit_slots(&self, base: &Self) {
         macro_rules! copyslot {
             ($slot:ident) => {
@@ -472,6 +474,28 @@ impl PyType {
             };
         }
 
+        // Copy init slot only if base actually defines it (not just inherited)
+        // This is needed for multiple inheritance where a later base might
+        // have a more specific init slot
+        macro_rules! copyslot_defined {
+            ($slot:ident) => {
+                if self.slots.$slot.load().is_none() {
+                    if let Some(base_val) = base.slots.$slot.load() {
+                        // SLOTDEFINED: base->SLOT && (basebase == NULL || base->SLOT != basebase->SLOT)
+                        let basebase = base.base.as_ref();
+                        let slot_defined = match basebase {
+                            None => true,
+                            Some(bb) => bb.slots.$slot.load().map(|v| v as usize)
+                                != Some(base_val as usize),
+                        };
+                        if slot_defined {
+                            self.slots.$slot.store(Some(base_val));
+                        }
+                    }
+                }
+            };
+        }
+
         // Core slots
         copyslot!(hash);
         copyslot!(call);
@@ -484,9 +508,8 @@ impl PyType {
         copyslot!(iternext);
         copyslot!(descr_get);
         copyslot!(descr_set);
-        // Note: init is NOT inherited here because object_init has special
-        // handling in CPython (checks if type->tp_init != object_init).
-        // TODO: implement proper init inheritance with object_init check
+        // init uses SLOTDEFINED check for multiple inheritance support
+        copyslot_defined!(init);
         copyslot!(del);
         // new is handled by set_new()
         // as_buffer is inherited at type creation time (not AtomicCell)
@@ -832,7 +855,16 @@ impl Py<PyType> {
 }
 
 #[pyclass(
-    with(Py, Constructor, GetAttr, SetAttr, Callable, AsNumber, Representable),
+    with(
+        Py,
+        Constructor,
+        Initializer,
+        GetAttr,
+        SetAttr,
+        Callable,
+        AsNumber,
+        Representable
+    ),
     flags(BASETYPE)
 )]
 impl PyType {
@@ -1532,6 +1564,26 @@ fn get_doc_from_internal_doc<'a>(name: &str, internal_doc: &'a str) -> &'a str {
     internal_doc
 }
 
+impl Initializer for PyType {
+    type Args = FuncArgs;
+
+    // type_init
+    fn slot_init(_zelf: PyObjectRef, args: FuncArgs, vm: &VirtualMachine) -> PyResult<()> {
+        // type.__init__() takes 1 or 3 arguments
+        if args.args.len() == 1 && !args.kwargs.is_empty() {
+            return Err(vm.new_type_error("type.__init__() takes no keyword arguments".to_owned()));
+        }
+        if args.args.len() != 1 && args.args.len() != 3 {
+            return Err(vm.new_type_error("type.__init__() takes 1 or 3 arguments".to_owned()));
+        }
+        Ok(())
+    }
+
+    fn init(_zelf: PyRef<Self>, _args: Self::Args, _vm: &VirtualMachine) -> PyResult<()> {
+        unreachable!("slot_init is defined")
+    }
+}
+
 impl GetAttr for PyType {
     fn getattro(zelf: &Py<Self>, name_str: &Py<PyStr>, vm: &VirtualMachine) -> PyResult {
         #[cold]
diff --git a/crates/vm/src/exception_group.rs b/crates/vm/src/exception_group.rs
index 645a3e779f..b2dee20620 100644
--- a/crates/vm/src/exception_group.rs
+++ b/crates/vm/src/exception_group.rs
@@ -353,21 +353,13 @@ pub(super) mod types {
     impl Initializer for PyBaseExceptionGroup {
         type Args = FuncArgs;
 
-        fn slot_init(
-            _zelf: PyObjectRef,
-            _args: ::rustpython_vm::function::FuncArgs,
-            _vm: &::rustpython_vm::VirtualMachine,
-        ) -> ::rustpython_vm::PyResult<()> {
-            // CPython's BaseExceptionGroup.__init__ just calls BaseException.__init__
-            // which stores args as-is. Since __new__ already set up the correct args
-            // (message, exceptions_tuple), we don't need to do anything here.
-            // This also allows subclasses to pass extra arguments to __new__ without
-            // __init__ complaining about argument count.
+        fn slot_init(_zelf: PyObjectRef, _args: FuncArgs, _vm: &VirtualMachine) -> PyResult<()> {
+            // No-op: __new__ already set up the correct args (message, exceptions_tuple)
             Ok(())
         }
 
         fn init(_zelf: PyRef<Self>, _args: Self::Args, _vm: &VirtualMachine) -> PyResult<()> {
-            unreachable!("slot_init is defined")
+            unreachable!("slot_init is overridden")
         }
     }
 
diff --git a/crates/vm/src/stdlib/io.rs b/crates/vm/src/stdlib/io.rs
index 9402660a86..a02965797b 100644
--- a/crates/vm/src/stdlib/io.rs
+++ b/crates/vm/src/stdlib/io.rs
@@ -3679,23 +3679,31 @@ mod _io {
     }
 
     impl Constructor for StringIO {
+        type Args = FuncArgs;
+
+        fn py_new(_cls: &Py<PyType>, _args: Self::Args, _vm: &VirtualMachine) -> PyResult<Self> {
+            Ok(Self {
+                _base: Default::default(),
+                buffer: PyRwLock::new(BufferedIO::new(Cursor::new(Vec::new()))),
+                closed: AtomicCell::new(false),
+            })
+        }
+    }
+
+    impl Initializer for StringIO {
         type Args = StringIONewArgs;
 
         #[allow(unused_variables)]
-        fn py_new(
-            _cls: &Py<PyType>,
+        fn init(
+            zelf: PyRef<Self>,
             Self::Args { object, newline }: Self::Args,
             _vm: &VirtualMachine,
-        ) -> PyResult<Self> {
+        ) -> PyResult<()> {
             let raw_bytes = object
                 .flatten()
                 .map_or_else(Vec::new, |v| v.as_bytes().to_vec());
-
-            Ok(Self {
-                _base: Default::default(),
-                buffer: PyRwLock::new(BufferedIO::new(Cursor::new(raw_bytes))),
-                closed: AtomicCell::new(false),
-            })
+            *zelf.buffer.write() = BufferedIO::new(Cursor::new(raw_bytes));
+            Ok(())
         }
     }
 
@@ -3709,7 +3717,7 @@ mod _io {
         }
     }
 
-    #[pyclass(flags(BASETYPE, HAS_DICT), with(Constructor))]
+    #[pyclass(flags(BASETYPE, HAS_DICT), with(Constructor, Initializer))]
     impl StringIO {
         #[pymethod]
         const fn readable(&self) -> bool {
@@ -3814,22 +3822,35 @@ mod _io {
     }
 
     impl Constructor for BytesIO {
-        type Args = OptionalArg<Option<PyBytesRef>>;
-
-        fn py_new(_cls: &Py<PyType>, object: Self::Args, _vm: &VirtualMachine) -> PyResult<Self> {
-            let raw_bytes = object
-                .flatten()
-                .map_or_else(Vec::new, |input| input.as_bytes().to_vec());
+        type Args = FuncArgs;
 
+        fn py_new(_cls: &Py<PyType>, _args: Self::Args, _vm: &VirtualMachine) -> PyResult<Self> {
             Ok(Self {
                 _base: Default::default(),
-                buffer: PyRwLock::new(BufferedIO::new(Cursor::new(raw_bytes))),
+                buffer: PyRwLock::new(BufferedIO::new(Cursor::new(Vec::new()))),
                 closed: AtomicCell::new(false),
                 exports: AtomicCell::new(0),
             })
         }
     }
 
+    impl Initializer for BytesIO {
+        type Args = OptionalArg<Option<ArgBytesLike>>;
+
+        fn init(zelf: PyRef<Self>, object: Self::Args, vm: &VirtualMachine) -> PyResult<()> {
+            if zelf.exports.load() > 0 {
+                return Err(vm.new_buffer_error(
+                    "Existing exports of data: object cannot be re-sized".to_owned(),
+                ));
+            }
+            let raw_bytes = object
+                .flatten()
+                .map_or_else(Vec::new, |input| input.borrow_buf().to_vec());
+            *zelf.buffer.write() = BufferedIO::new(Cursor::new(raw_bytes));
+            Ok(())
+        }
+    }
+
     impl BytesIO {
         fn buffer(&self, vm: &VirtualMachine) -> PyResult<PyRwLockWriteGuard<'_, BufferedIO>> {
             if !self.closed.load() {
@@ -3840,7 +3861,7 @@ mod _io {
         }
     }
 
-    #[pyclass(flags(BASETYPE, HAS_DICT), with(PyRef, Constructor))]
+    #[pyclass(flags(BASETYPE, HAS_DICT), with(PyRef, Constructor, Initializer))]
     impl BytesIO {
         #[pymethod]
         const fn readable(&self) -> bool {
diff --git a/crates/vm/src/types/slot.rs b/crates/vm/src/types/slot.rs
index bfe6e04762..13a43ed9c9 100644
--- a/crates/vm/src/types/slot.rs
+++ b/crates/vm/src/types/slot.rs
@@ -632,13 +632,48 @@ impl PyType {
                 update_slot!(descr_set, descr_set_wrapper);
             }
             _ if name == identifier!(ctx, __init__) => {
-                toggle_slot!(init, init_wrapper);
+                // Special handling: check if this type or any base has a real __init__ method
+                // If only slot wrappers exist, use the inherited slot from copyslot!
+                if ADD {
+                    // First check if this type has __init__ in its own dict
+                    let has_own_init = self.attributes.read().contains_key(name);
+                    if has_own_init {
+                        // This type defines __init__ - use wrapper
+                        self.slots.init.store(Some(init_wrapper));
+                    } else if self.has_real_method_in_mro(name, ctx) {
+                        // A base class defines a real __init__ method - use wrapper
+                        self.slots.init.store(Some(init_wrapper));
+                    }
+                    // else: keep inherited slot from copyslot!
+                } else {
+                    let inherited = self
+                        .mro
+                        .read()
+                        .iter()
+                        .skip(1)
+                        .find_map(|cls| cls.slots.init.load());
+                    self.slots.init.store(inherited);
+                }
             }
             _ if name == identifier!(ctx, __new__) => {
                 toggle_slot!(new, new_wrapper);
             }
             _ if name == identifier!(ctx, __del__) => {
-                toggle_slot!(del, del_wrapper);
+                // Same special handling as __init__
+                if ADD {
+                    let has_own_del = self.attributes.read().contains_key(name);
+                    if has_own_del || self.has_real_method_in_mro(name, ctx) {
+                        self.slots.del.store(Some(del_wrapper));
+                    }
+                } else {
+                    let inherited = self
+                        .mro
+                        .read()
+                        .iter()
+                        .skip(1)
+                        .find_map(|cls| cls.slots.del.load());
+                    self.slots.del.store(inherited);
+                }
             }
             _ if name == identifier!(ctx, __bool__) => {
                 toggle_sub_slot!(as_number, boolean, bool_wrapper);
@@ -890,6 +925,36 @@ impl PyType {
             _ => {}
         }
     }
+
+    /// Check if there's a real method (not a slot wrapper) for `name` anywhere in the MRO.
+    /// If a real method exists, we should use a wrapper function. Otherwise, use the inherited slot.
+    fn has_real_method_in_mro(&self, name: &'static PyStrInterned, ctx: &Context) -> bool {
+        use crate::builtins::descriptor::PySlotWrapper;
+
+        // Check the entire MRO (including self) for the method
+        for cls in self.mro.read().iter() {
+            if let Some(attr) = cls.attributes.read().get(name).cloned() {
+                // Found the method - check if it's a slot wrapper
+                if attr.class().is(ctx.types.wrapper_descriptor_type) {
+                    if let Some(wrapper) = attr.downcast_ref::<PySlotWrapper>() {
+                        // It's a slot wrapper - check if it belongs to this class
+                        let wrapper_typ: *const _ = wrapper.typ;
+                        let cls_ptr: *const _ = cls.as_ref();
+                        if wrapper_typ == cls_ptr {
+                            // Slot wrapper defined on this exact class - use inherited slot
+                            return false;
+                        }
+                    }
+                    // Inherited slot wrapper - continue checking MRO
+                } else {
+                    // Real method found - use wrapper function
+                    return true;
+                }
+            }
+        }
+        // No real method found in MRO - use inherited slot
+        false
+    }
 }
 
 /// Trait for types that can be constructed via Python's `__new__` method.
diff --git a/crates/vm/src/types/zoo.rs b/crates/vm/src/types/zoo.rs
index 0cd04a0ac1..9d16424167 100644
--- a/crates/vm/src/types/zoo.rs
+++ b/crates/vm/src/types/zoo.rs
@@ -199,8 +199,10 @@ impl TypeZoo {
     /// Fill attributes of builtin types.
     #[cold]
     pub(crate) fn extend(context: &Context) {
-        type_::init(context);
+        // object must be initialized before type to set object.slots.init,
+        // which type will inherit via inherit_slots()
         object::init(context);
+        type_::init(context);
         list::init(context);
         set::init(context);
         tuple::init(context);