Allow js functions as python callbacks

src/python.ts

@@ -5,6 +5,14 @@ import { cstr, SliceItemRegExp } from "./util.ts";
 
 const refregistry = new FinalizationRegistry(py.Py_DecRef);
 
+// FinalizationRegistry for auto-created callbacks
+// Closes the callback when the PyObject holding it is GC'd
+const callbackCleanupRegistry = new FinalizationRegistry(
+  (callback: Callback) => {
+    callback.destroy();
+  },
+);
+
 /**
  * Symbol used on proxied Python objects to point to the original PyObject object.
  * Can be used to implement PythonProxy and create your own proxies.
@@ -548,6 +556,7 @@ export class PyObject {
           // Is this still needed (after the change of pinning fields to the callabck) ? might be
           const pyObject = new PyObject(fn);
           pyObject.#pyMethodDef = methodDef;
+          // Note: explicit Callback objects are user-managed, not auto-cleaned
           return pyObject;
         } else if (v instanceof PyObject) {
           return v;
@@ -601,6 +610,14 @@ export class PyObject {
         if (ProxiedPyObject in v) {
           return (v as any)[ProxiedPyObject];
         }
+
+        if (typeof v === "function") {
+          const callback = new Callback(v);
+          const pyObject = PyObject.from(callback);
+          // Register cleanup to close callback when PyObject is GC'd
+          callbackCleanupRegistry.register(pyObject, callback);
+          return pyObject;
+        }
       }
 
       default:

test/test_with_gc.ts

@@ -1,6 +1,22 @@
-import python, { Callback } from "../mod.ts";
+import python, { Callback, PyObject } from "../mod.ts";
 import { assertEquals } from "./asserts.ts";
 
+Deno.test("js fns are automaticlly converted to callbacks", () => {
+  const pyModule = python.runModule(
+    `
+def call_the_callback(cb):
+  result = cb()
+  return result + 1
+  `,
+    "test_module",
+  );
+
+  assertEquals(pyModule.call_the_callback(() => 4).valueOf(), 5);
+
+  // @ts-ignore:requires: --v8-flags=--expose-gc
+  gc(); // if this is commented out, the test will fail beacuse the callback was not freed
+});
+
 Deno.test("callbacks are not gc'd while still needed by python", () => {
   const pyModule = python.runModule(
     `
@@ -46,3 +62,64 @@ def call_stored_callback():
   assertEquals(callCount, 3);
   callbackObj.destroy();
 });
+
+Deno.test("callbacks are not gc'd while still needed by python (function version)", () => {
+  const pyModule = python.runModule(
+    `
+stored_callback = None
+
+def store_and_call_callback(cb):
+  global stored_callback
+  stored_callback = cb
+  return stored_callback()
+
+def call_stored_callback():
+  global stored_callback
+  if stored_callback is None:
+    return -1
+  return stored_callback()
+  `,
+    "test_gc_module",
+  );
+
+  let callCount = 0;
+  const callback = () => {
+    callCount++;
+    return callCount * 10;
+  };
+
+  // Store the callback in Python and call it
+  assertEquals(pyModule.store_and_call_callback(callback).valueOf(), 10);
+  assertEquals(callCount, 1);
+
+  for (let i = 0; i < 10; i++) {
+    // @ts-ignore:requires: --v8-flags=--expose-gc
+    gc();
+  }
+
+  // If the callback was incorrectly GC'd, this should segfault
+  // But it should work because Python holds a reference
+  assertEquals(pyModule.call_stored_callback().valueOf(), 20);
+  assertEquals(callCount, 2);
+
+  // Call it again to be sure
+  assertEquals(pyModule.call_stored_callback().valueOf(), 30);
+  assertEquals(callCount, 3);
+});
+
+Deno.test("auto-created callbacks are cleaned up after gc", () => {
+  // Create callback and explicitly null it out to help GC
+  // @ts-ignore PyObject can be created from fns its just the types are not exposed
+  // deno-lint-ignore no-explicit-any
+  let _f: any = PyObject.from(() => 5);
+
+  // Explicitly null the reference
+  _f = null;
+
+  // Now f is null, trigger GC to clean it up
+  // Run many GC cycles with delays to ensure finalizers execute
+  for (let i = 0; i < 10; i++) {
+    // @ts-ignore:requires: --v8-flags=--expose-gc
+    gc();
+  }
+});