RustPython · youknowone · Aug 14, 2026 · Aug 13, 2026
diff --git a/crates/vm/src/py_serde.rs b/crates/vm/src/py_serde.rs
@@ -63,7 +63,10 @@ impl serde::Serialize for PyObjectSerializer<'_> {
                 seq.end()
             };
         if let Some(s) = self.pyobject.downcast_ref::<PyStr>() {
-            serializer.serialize_str(s.as_ref())
+            serializer.serialize_str(
+                s.to_str()
+                    .ok_or_else(|| serde::ser::Error::custom("str contains surrogates"))?,
+            )
         } else if self.pyobject.fast_isinstance(self.vm.ctx.types.float_type) {
             serializer.serialize_f64(float::get_value(self.pyobject))
         } else if self.pyobject.fast_isinstance(self.vm.ctx.types.bool_type) {

diff --git a/crates/wasm/src/convert.rs b/crates/wasm/src/convert.rs
@@ -2,17 +2,40 @@
 
 use crate::js_module;
 use crate::vm_class::{WASMVirtualMachine, stored_vm_from_wasm};
-use js_sys::{Array, ArrayBuffer, Object, Promise, Reflect, SyntaxError, Uint8Array};
+use js_sys::{
+    Array, ArrayBuffer, JsString, Map, Object, Promise, Reflect, SyntaxError, Uint8Array,
+};
+use rustpython_common::wtf8::{Wtf8, Wtf8Buf};
 use rustpython_vm::{
     AsObject, Py, PyObjectRef, PyPayload, PyResult, TryFromBorrowedObject, VirtualMachine,
-    builtins::{PyBaseException, PyBaseExceptionRef},
+    builtins::{PyBaseException, PyBaseExceptionRef, PyDict, PyList, PyStr, PyTuple},
     compiler::{CompileError, ParseError, parser::LexicalErrorType, parser::ParseErrorType},
     exceptions,
     function::{ArgBytesLike, FuncArgs},
     py_serde,
 };
 use wasm_bindgen::{JsCast, closure::Closure, prelude::*};
 
+pub(crate) fn js_string_to_wtf8(value: &JsString) -> Wtf8Buf {
+    Wtf8Buf::from_wide(&value.iter().collect::<Vec<_>>())
+}
+
+fn wtf8_to_js_string(value: &Wtf8) -> JsString {
+    const CHUNK_SIZE: usize = 8192;
+
+    if let Ok(value) = value.as_str() {
+        return value.into();
+    }
+
+    value
+        .encode_wide()
+        .collect::<Vec<_>>()
+        .chunks(CHUNK_SIZE)
+        .map(JsString::from_char_code)
+        .collect::<Array>()
+        .join("")
+}
+
 #[wasm_bindgen(inline_js = r"
 export class PyError extends Error {
     constructor(info) {
@@ -119,12 +142,9 @@ pub fn py_to_js(vm: &VirtualMachine, py_obj: PyObjectRef) -> JsValue {
                 if let Some(ref kwargs) = kwargs {
                     for pair in object_entries(kwargs) {
                         let (key, val) = pair?;
-                        py_func_args.kwargs.insert(
-                            // JS strings coming in are UTF-16; go through Rust `String`
-                            // (kwargs keys are now WTF-8, so convert String -> Wtf8Buf).
-                            String::from(js_sys::JsString::from(key)).into(),
-                            js_to_py(vm, val),
-                        );
+                        py_func_args
+                            .kwargs
+                            .insert(js_string_to_wtf8(&key.into()), js_to_py(vm, val));
                     }
                 }
                 let result = py_obj.call(py_func_args, vm);
@@ -151,17 +171,44 @@ pub fn py_to_js(vm: &VirtualMachine, py_obj: PyObjectRef) -> JsValue {
     }
 
     if let Ok(bytes) = ArgBytesLike::try_from_borrowed_object(vm, &py_obj) {
-        bytes.with_ref(|bytes| unsafe {
+        return bytes.with_ref(|bytes| unsafe {
             // `Uint8Array::view` is an `unsafe fn` because it provides
             // a direct view into the WASM linear memory; if you were to allocate
             // something with Rust that view would probably become invalid. It's safe
             // because we then copy the array using `Uint8Array::slice`.
             let view = Uint8Array::view(bytes);
             view.slice(0, bytes.len() as u32).into()
-        })
+        });
+    }
+    py_serde_to_js(vm, &py_obj).unwrap_or(JsValue::UNDEFINED)
+}
+
+fn py_serde_to_js(
+    vm: &VirtualMachine,
+    py_obj: &PyObjectRef,
+) -> Result<JsValue, serde_wasm_bindgen::Error> {
+    if let Some(value) = py_obj.downcast_ref::<PyStr>() {
+        Ok(wtf8_to_js_string(value.as_wtf8()).into())
+    } else if let Some(value) = py_obj.downcast_ref::<PyList>() {
+        let array = Array::new();
+        for item in value.borrow_vec().iter() {
+            array.push(&py_serde_to_js(vm, item)?);
+        }
+        Ok(array.into())
+    } else if let Some(value) = py_obj.downcast_ref::<PyTuple>() {
+        let array = Array::new();
+        for item in value {
+            array.push(&py_serde_to_js(vm, item)?);
+        }
+        Ok(array.into())
+    } else if let Some(value) = py_obj.downcast_ref::<PyDict>() {
+        let map = Map::new();
+        for (key, value) in value {
+            map.set(&py_serde_to_js(vm, &key)?, &py_serde_to_js(vm, &value)?);
+        }
+        Ok(map.into())
     } else {
-        py_serde::serialize(vm, &py_obj, &serde_wasm_bindgen::Serializer::new())
-            .unwrap_or(JsValue::UNDEFINED)
+        py_serde::serialize(vm, py_obj, &serde_wasm_bindgen::Serializer::new())
     }
 }
 
@@ -199,6 +246,15 @@ pub fn js_to_py(vm: &VirtualMachine, js_val: JsValue) -> PyObjectRef {
                 .map(|val| js_to_py(vm, val.expect("Iteration over array failed")))
                 .collect();
             vm.ctx.new_list(elems).into()
+        } else if let Some(map) = js_val.dyn_ref::<Map>() {
+            let dict = vm.ctx.new_dict();
+            for entry in map.entries() {
+                let entry = Array::from(&entry.expect("Iteration over map failed"));
+                let key = js_to_py(vm, entry.get(0));
+                dict.set_item(&*key, js_to_py(vm, entry.get(1)), vm)
+                    .unwrap();
+            }
+            dict.into()
         } else if ArrayBuffer::is_view(&js_val) || js_val.is_instance_of::<ArrayBuffer>() {
             // unchecked_ref because if it's not an ArrayBuffer it could either be a TypedArray
             // or a DataView, but they all have a `buffer` property
@@ -216,12 +272,8 @@ pub fn js_to_py(vm: &VirtualMachine, js_val: JsValue) -> PyObjectRef {
             for pair in object_entries(&Object::from(js_val)) {
                 let (key, val) = pair.expect("iteration over object to not fail");
                 let py_val = js_to_py(vm, val);
-                dict.set_item(
-                    String::from(js_sys::JsString::from(key)).as_str(),
-                    py_val,
-                    vm,
-                )
-                .unwrap();
+                dict.set_item(&*js_string_to_wtf8(&key.into()), py_val, vm)
+                    .unwrap();
             }
             dict.into()
         }
@@ -232,9 +284,7 @@ pub fn js_to_py(vm: &VirtualMachine, js_val: JsValue) -> PyObjectRef {
             move |args: FuncArgs, vm: &VirtualMachine| -> PyResult {
                 let this = Object::new();
                 for (k, v) in args.kwargs {
-                    // WTF-8 -> JS string: lone surrogates in the key become U+FFFD
-                    // (wasm-bindgen only accepts Rust `String`); acceptable at this boundary.
-                    Reflect::set(&this, &k.to_string().into(), &py_to_js(vm, v))
+                    Reflect::set(&this, &wtf8_to_js_string(&k).into(), &py_to_js(vm, v))
                         .expect("property to be settable");
                 }
                 let js_args = args
@@ -253,6 +303,8 @@ pub fn js_to_py(vm: &VirtualMachine, js_val: JsValue) -> PyObjectRef {
     } else if js_val.is_undefined() {
         // Because `JSON.stringify(undefined)` returns undefined
         vm.ctx.none()
+    } else if js_val.is_string() {
+        vm.ctx.new_str(js_string_to_wtf8(&js_val.into())).into()
     } else {
         py_serde::deserialize(vm, serde_wasm_bindgen::Deserializer::from(js_val))
             .unwrap_or_else(|_| vm.ctx.none())

diff --git a/crates/wasm/src/vm_class.rs b/crates/wasm/src/vm_class.rs
@@ -325,9 +325,12 @@ impl WASMVirtualMachine {
             if let Some(imports) = imports {
                 for entry in convert::object_entries(&imports) {
                     let (key, value) = entry?;
-                    let key: String = Object::from(key).to_string().into();
                     attrs
-                        .set_item(key.as_str(), convert::js_to_py(vm, value), vm)
+                        .set_item(
+                            &*convert::js_string_to_wtf8(&key.into()),
+                            convert::js_to_py(vm, value),
+                            vm,
+                        )
                         .into_js(vm)?;
                 }
             }
@@ -356,10 +359,10 @@ impl WASMVirtualMachine {
             let py_module = vm.new_module(&name, vm.ctx.new_dict(), None);
             for entry in convert::object_entries(&module) {
                 let (key, value) = entry?;
-                let key = Object::from(key).to_string();
-                extend_module!(vm, &py_module, {
-                    String::from(key) => convert::js_to_py(vm, value),
-                });
+                let key = vm.ctx.new_str(convert::js_string_to_wtf8(&key.into()));
+                py_module
+                    .set_attr(&key, convert::js_to_py(vm, value), vm)
+                    .into_js(vm)?;
             }
 
             let sys_modules = vm.sys_module.get_attr("modules", vm).into_js(vm)?;