Add destroy method to remove references to Python objects

docs/type_conversions.md

@@ -82,9 +82,16 @@ currently supported:
 
 An additional limitation is that when passing a Python object to Javascript,
 there is no way for Javascript to automatically garbage collect that object.
-Therefore, Python objects must be manually free'd when passed to Javascript, or
-they will leak. (TODO: There isn't currently a way to do this, but it will be
-implemented soon).
+Therefore, custom Python objects must be manually destroyed when passed to Javascript, or
+they will leak. To do this, call `.destroy()` on the object, after which Javascript will no longer have access to the object.
+
+```javascript
+var foo = pyodide.pyimport('foo');
+foo.call_method();
+foo.destroy();
+foo.call_method(); // This will raise an exception, since the object has been
+                   // destroyed
+```
 
 ## Using Python objects from Javascript
 

src/pyproxy.c

@@ -116,6 +116,13 @@ _pyproxy_apply(int ptrobj, int idargs)
   return idresult;
 }
 
+void
+_pyproxy_destroy(int ptrobj)
+{
+  PyObject* pyobj = (PyObject*)ptrobj;
+  Py_DECREF(ptrobj);
+}
+
 EM_JS(int, pyproxy_new, (int ptrobj), {
   var target = function(){};
   target['$$'] = { ptr : ptrobj, type : 'PyProxy' };
@@ -126,7 +133,11 @@ EM_JS(int, pyproxy_init, (), {
   // clang-format off
   Module.PyProxy = {
     getPtr: function(jsobj) {
-      return jsobj['$$']['ptr'];
+      var ptr = jsobj['$$']['ptr'];
+      if (ptr === null) {
+        throw new Error("Object has already been destroyed");
+      }
+      return ptr;
     },
     isPyProxy: function(jsobj) {
       return jsobj['$$'] !== undefined && jsobj['$$']['type'] === 'PyProxy';
@@ -149,6 +160,9 @@ EM_JS(int, pyproxy_init, (), {
         }
       } else if (jskey === '$$') {
         return jsobj['$$'];
+      } else if (jskey === 'destroy') {
+        __pyproxy_destroy(this.getPtr(jsobj));
+        jsobj['$$']['ptr'] = null;
       }
       ptrobj = this.getPtr(jsobj);
       var idkey = Module.hiwire_new_value(jskey);

test/test_python.py

@@ -52,6 +52,8 @@ def test_pythonexc2js(selenium):
         selenium.run_js('return pyodide.runPython("5 / 0")')
     except JavascriptException as e:
         assert('ZeroDivisionError' in str(e))
+    else:
+        assert False, 'Expected exception'
 
 
 def test_js2python(selenium):
@@ -131,6 +133,26 @@ def test_pyproxy(selenium):
         "return pyodide.pyimport('f').toString()").startswith('<Foo')
 
 
+def test_pyproxy_destroy(selenium):
+    selenium.run(
+        "class Foo:\n"
+        "  bar = 42\n"
+        "  def get_value(self, value):\n"
+        "    return value * 64\n"
+        "f = Foo()\n"
+    )
+    try:
+        selenium.run_js(
+            "let f = pyodide.pyimport('f');\n"
+            "console.assert(f.get_value(1) === 64);\n"
+            "f.destroy();\n"
+            "f.get_value();\n")
+    except JavascriptException as e:
+        assert 'f.get_value is not a function' in str(e)
+    else:
+        assert False, 'Expected exception'
+
+
 def test_jsproxy(selenium):
     assert selenium.run(
         "from js import document\n"