Add store_item() virtual method to type to implement container[index] = val.

py/obj.h

@@ -97,6 +97,7 @@ typedef mp_obj_t (*mp_unary_op_fun_t)(int op, mp_obj_t);
 typedef mp_obj_t (*mp_binary_op_fun_t)(int op, mp_obj_t, mp_obj_t);
 typedef void (*mp_load_attr_fun_t)(mp_obj_t self_in, qstr attr, mp_obj_t *dest); // for fail, do nothing; for attr, dest[1] = value; for method, dest[0] = self, dest[1] = method
 typedef bool (*mp_store_attr_fun_t)(mp_obj_t self_in, qstr attr, mp_obj_t value); // return true if store succeeded
+typedef bool (*mp_store_item_fun_t)(mp_obj_t self_in, mp_obj_t index, mp_obj_t value); // return true if store succeeded
 
 typedef struct _mp_method_t {
     const char *name;
@@ -162,6 +163,9 @@ struct _mp_obj_type_t {
 
     mp_load_attr_fun_t load_attr;
     mp_store_attr_fun_t store_attr;
+    // Implements container[index] = val; note that load_item is implemented
+    // by binary_op(RT_BINARY_OP_SUBSCR)
+    mp_store_item_fun_t store_item;
 
     // these are for dynamically created types (classes)
     mp_obj_t bases_tuple;

py/runtime.c

@@ -905,7 +905,15 @@ void rt_store_subscr(mp_obj_t base, mp_obj_t index, mp_obj_t value) {
         // dict store
         mp_obj_dict_store(base, index, value);
     } else {
-        assert(0);
+        mp_obj_type_t *type = mp_obj_get_type(base);
+        if (type->store_item != NULL) {
+            bool r = type->store_item(base, index, value);
+            if (r) {
+                return;
+            }
+            // TODO: call base classes here?
+        }
+        nlr_jump(mp_obj_new_exception_msg_varg(MP_QSTR_TypeError, "'%s' object does not support item assignment", mp_obj_get_type_str(base)));
     }
 }