Type-check arg to Sprite.touching()

Previously, code like

    elephant.touching("Giraffe")

gave an unhelpful "internal error"-type message.  Instead, type-check
the arg and provide a hopefully more helpful error if invalid.

Author: bennorth

src/lib/pytch/actor.py

@@ -2,6 +2,7 @@
     play_sound,
     _get_actor_sound_mix_bus_gain,
     _set_actor_sound_mix_bus_gain,
+    _is_Pytch_registered_Sprite,
     registered_instances,
     unregister_running_instance,
     wait_seconds,
@@ -359,6 +360,11 @@ def costume_name(self):
 
     def touching(self, target_class):
         "(TARGET) Return whether SELF touches any TARGET instance"
+        if not _is_Pytch_registered_Sprite(target_class):
+            raise TypeError(
+                "in touching(target_class), target_class must be"
+                " a Pytch-registered Sprite class"
+            )
         return (self._pytch_parent_project
                 .instance_is_touching_any_of(self, target_class))
 

src/lib/pytch/syscalls.js

@@ -289,6 +289,26 @@ var $builtinmodule = function (name) {
         `(SECONDS) Pause for the given number of seconds`,
     );
 
+    mod._is_Pytch_registered_Sprite = skulpt_function(
+	(obj) => {
+	    if (!Sk.builtin.checkClass(obj))
+		return Sk.builtin.bool.false$;
+
+            const actor = obj.$pytchActor
+            const is_Sprite = (
+                actor != null
+                && actor.class_kind_name === "Sprite"
+	    )
+
+	    return (
+		is_Sprite
+                    ? Sk.builtin.bool.true$
+                    : Sk.builtin.bool.false$
+            );
+	},
+	"Whether the given OBJ is the class object for a Sprite"
+    );
+
     mod._effective_source_object = skulpt_function(
         (py_cls_or_obj) => {
             if (Sk.builtin.checkClass(py_cls_or_obj)) {

test/pytch/test_collision_detection.js

@@ -6,6 +6,9 @@ const {
     assert,
     assert_has_bbox,
     call_method,
+    import_deindented,
+    one_frame,
+    pytch_errors,
 } = require("./pytch-testing.js");
 configure_mocha();
 
@@ -15,6 +18,33 @@ configure_mocha();
 // Bounding box computation, collision detection.
 
 describe("collision detection", () => {
+    [
+        { label: "string", expr: '"Giraffe"' },
+        { label: "number", expr: '42' },
+        { label: "Stage", expr: 'AnimalStage' },
+        { label: "non-Sprite class", expr: 'EmptyClass' },
+    ].forEach(spec =>
+        it(`rejects touching() of ${spec.label}`, async () => {
+            const project = await import_deindented(`
+                import pytch
+                class EmptyClass: pass
+                class AnimalStage(pytch.Stage):
+                    Backdrops = ["solid-white-stage.png"]
+                class Elephant(pytch.Sprite):
+                    @pytch.when_I_receive("go")
+                    def try_go(self):
+                        if self.touching(${spec.expr}):
+                            pass
+            `);
+
+            project.do_synthetic_broadcast("go")
+            one_frame(project);
+            pytch_errors.assert_sole_error_matches(
+                /target_class must be.*Sprite class/
+            );
+        })
+    );
+
     with_project("py/project/bounding_boxes.py", (import_project) => {
         it("can extract bounding boxes", async () => {
             let project = await import_project();