Fix leak in to_js of dictionary (#4853)

docs/project/changelog.md

@@ -16,6 +16,9 @@ myst:
 
 ## Unreleased
 
+- {{ Fix }} Don't leak the values in a dictionary when applying `to_js` to it.
+  {pr}`4853`
+
 ### Packages
 
 - Upgraded `scikit-learn` to 1.5 {pr}`4823`

src/core/python2js.c

@@ -275,6 +275,7 @@ _python2js_dict(ConversionContext* context, PyObject* x)
       FAIL_IF_MINUS_ONE(
         context->dict_add_keyvalue(context, jsdict, jskey, jsval));
     }
+    Py_CLEAR(item);
   }
   FAIL_IF_ERR_OCCURRED();
   if (context->dict_postprocess) {

src/tests/test_typeconversions.py

@@ -2200,3 +2200,12 @@ def test_bind_construct(selenium):
     A = A.bind_sig(A_sig)
 
     A()
+
+
+@run_in_pyodide
+def test_to_js_no_leak(selenium):
+    from js import Object
+    from pyodide.ffi import to_js
+
+    d = {"key": Object()}
+    to_js(d)