Add descr_get handler to JsMethod (#3130)

docs/project/changelog.md

@@ -54,6 +54,12 @@ substitutions:
   first argument using the {any}`captureThis` method.
   {pr}`3103`
 
+- {{ Enhancement }} A `JsProxy` of a function now has a `__get__` descriptor
+  method, so it's possible to use a JavaScript function as a Python method. When
+  the method is called, `this` will be a `PyProxy` pointing to the Python object
+  the method is called on.
+  {pr}`3130`
+
 ### Build System / Package Loading
 
 - New packages: pycryptodomex {pr}`2966`, pycryptodome {pr}`2965`,

src/core/jsproxy.c

@@ -103,6 +103,9 @@ typedef struct
 static void
 JsProxy_dealloc(JsProxy* self)
 {
+  if (pyproxy_Check(self->this_)) {
+    destroy_proxy(self->this_, NULL);
+  }
 #ifdef DEBUG_F
   extern bool tracerefs;
   if (tracerefs) {
@@ -1897,6 +1900,26 @@ static PyMethodDef JsMethod_Construct_MethodDef = {
 };
 // clang-format on
 
+static PyObject*
+JsMethod_descr_get(PyObject* self, PyObject* obj, PyObject* type)
+{
+  JsRef jsobj = NULL;
+  PyObject* result = NULL;
+
+  if (obj == Py_None || obj == NULL) {
+    Py_INCREF(self);
+    return self;
+  }
+
+  jsobj = python2js(obj);
+  FAIL_IF_NULL(jsobj);
+  result = JsProxy_create_with_this(JsProxy_REF(self), jsobj);
+
+finally:
+  hiwire_CLEAR(jsobj);
+  return result;
+}
+
 static int
 JsMethod_cinit(PyObject* obj, JsRef this_)
 {
@@ -2445,6 +2468,8 @@ JsProxy_create_subtype(int flags)
     tp_flags |= _Py_TPFLAGS_HAVE_VECTORCALL;
     slots[cur_slot++] =
       (PyType_Slot){ .slot = Py_tp_call, .pfunc = (void*)PyVectorcall_Call };
+    slots[cur_slot++] = (PyType_Slot){ .slot = Py_tp_descr_get,
+                                       .pfunc = (void*)JsMethod_descr_get };
     // We could test separately for whether a function is constructable,
     // but it generates a lot of false positives.
     methods[cur_method++] = JsMethod_Construct_MethodDef;

src/tests/test_jsproxy.py

@@ -1276,3 +1276,20 @@ def test_jsarray_reverse(selenium):
 
     assert a.to_py() == l
     assert b.to_bytes() == bytes(l)
+
+
+@run_in_pyodide
+def test_jsproxy_descr_get(selenium):
+    from pyodide.code import run_js
+
+    class T:
+        a: int
+        b: int
+        f = run_js("function f(x) {return this[x]; }; f")
+
+    t = T()
+    t.a = 7
+    t.b = 66
+    assert t.f("a") == 7
+    assert t.f("b") == 66
+    assert t.f("c") is None