feat: set python exception if an error happens in a js callback (#83)

* feat: set python exception if an error happens in a js callback

* remove extra log

* change pyobject -> null, gives slightly better error message

Author: sigmaSd

src/python.ts

@@ -154,13 +154,38 @@ export class Callback {
         args: Deno.PointerValue,
         kwargs: Deno.PointerValue,
       ) => {
-        return PyObject.from(callback(
-          kwargs === null ? {} : Object.fromEntries(
-            new PyObject(kwargs).asDict()
-              .entries(),
-          ),
-          ...(args === null ? [] : new PyObject(args).valueOf()),
-        )).handle;
+        let result: PythonConvertible;
+        // Prepare arguments for the JS callback
+        try {
+          // Prepare arguments for the JS callback
+          const jsKwargs = kwargs === null
+            ? {}
+            : Object.fromEntries(new PyObject(kwargs).asDict().entries());
+          const jsArgs = args === null ? [] : new PyObject(args).valueOf();
+
+          // Call the actual JS function
+          result = callback(jsKwargs, ...jsArgs);
+
+          // Convert the JS return value back to a Python object
+          return PyObject.from(result).handle;
+        } catch (e) {
+          // An error occurred in the JS callback.
+          // We need to set a Python exception and return NULL.
+
+          // Prepare the error message for Python
+          const errorMessage = e instanceof Error
+            ? `${e.name}: ${e.message}` // Include JS error type and message
+            : String(e); // Fallback for non-Error throws
+          const cErrorMessage = cstr(`JS Callback Error: ${errorMessage}`);
+
+          const errorTypeHandle =
+            python.builtins.RuntimeError[ProxiedPyObject].handle;
+
+          // Set the Python exception (type and message)
+          py.PyErr_SetString(errorTypeHandle, cErrorMessage);
+
+          return null;
+        }
       },
     );
   }

src/symbols.ts

@@ -39,6 +39,11 @@ export const SYMBOLS = {
     result: "void",
   },
 
+  PyErr_SetString: {
+    parameters: ["pointer", "buffer"], // type, message
+    result: "void",
+  },
+
   PyDict_New: {
     parameters: [],
     result: "pointer",

test/test.ts

@@ -360,3 +360,27 @@ Deno.test("callbacks have signature", async (t) => {
     fn.destroy();
   });
 });
+
+Deno.test("js exception inside python callback returns python exception", () => {
+  const pyCallback = python.callback(() => {
+    throw new Error("This is an intentional error from JS!");
+  });
+
+  const pyModule = python.runModule(
+    `
+def call_the_callback(cb):
+  result = cb()
+  return result
+  `,
+    "test_module",
+  );
+
+  try {
+    pyModule.call_the_callback(pyCallback);
+  } catch (e) {
+    // deno-lint-ignore no-explicit-any
+    assertEquals((e as any).name, "PythonError");
+  } finally {
+    pyCallback.destroy();
+  }
+});