Support single callback that do not get any number

boxes/__init__.py

@@ -391,6 +391,9 @@ class Boxes:
         self.ctx.save()
         self.moveTo(x, y)
         if callable(callback):
+            if number is None:
+                callback()
+            else:
                 callback(number)
         elif hasattr(callback, '__getitem__'):
             try: