py/objgenerator: Fix handling of None passed as 2nd arg to throw().

Fixes issue #4527.

1 files changed, 16 insertions, 1 deletions

diff --git a/py/objgenerator.c b/py/objgenerator.c
index c306bb0c5..92bafea6a 100644
--- a/py/objgenerator.c
+++ b/py/objgenerator.c
@@ -252,7 +252,22 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_2(gen_instance_send_obj, gen_instance_send);
 
 STATIC mp_obj_t gen_instance_close(mp_obj_t self_in);
 STATIC mp_obj_t gen_instance_throw(size_t n_args, const mp_obj_t *args) {
-    mp_obj_t exc = (n_args == 2) ? args[1] : args[2];
+    // The signature of this function is: throw(type[, value[, traceback]])
+    // CPython will pass all given arguments through the call chain and process them
+    // at the point they are used (native generators will handle them differently to
+    // user-defined generators with a throw() method).  To save passing multiple
+    // values, MicroPython instead does partial processing here to reduce it down to
+    // one argument and passes that through:
+    // - if only args[1] is given, or args[2] is given but is None, args[1] is
+    //   passed through (in the standard case it is an exception class or instance)
+    // - if args[2] is given and not None it is passed through (in the standard
+    //   case it would be an exception instance and args[1] its corresponding class)
+    // - args[3] is always ignored
+
+    mp_obj_t exc = args[1];
+    if (n_args > 2 && args[2] != mp_const_none) {
+        exc = args[2];
+    }
 
     mp_obj_t ret = gen_resume_and_raise(args[0], mp_const_none, exc);
     if (ret == MP_OBJ_STOP_ITERATION) {