Generators now return nil when exhausted instead of raising StopIteration.

compiler/util.py

@@ -77,11 +77,8 @@ class Writer(object):
     # Assume that body is aligned with goto labels.
     self.write(body)
     with self.indent_block():
-      if block_.is_generator:
-        self.write('return nil, πF.Raise('
-                   'πg.StopIterationType.ToObject(), nil, nil)')
-      else:
-        self.write('return πg.None, nil')
+      self.write('return {}, nil'.format(
+          'nil' if block_.is_generator else 'πg.None'))
     self.write('})')
 
   def write_import_block(self, imports):

runtime/generator.go

@@ -86,6 +86,9 @@ func (g *Generator) resume(f *Frame, sendValue *Object) (*Object, *BaseException
 	}
 	result, raised := g.block.execInternal(g.frame, sendValue)
 	g.mutex.Lock()
+	if result == nil && raised == nil {
+		raised = f.Raise(StopIterationType.ToObject(), nil, nil)
+	}
 	if raised == nil {
 		g.state = generatorStateReady
 	} else {

runtime/generator_test.go

@@ -29,7 +29,7 @@ func TestGeneratorNext(t *testing.T) {
 	})
 	recursive = NewGenerator(recursiveBlock, NewDict()).ToObject()
 	empty := NewBlock(func(f *Frame, _ *Object) (*Object, *BaseException) {
-		return nil, f.Raise(StopIterationType.ToObject(), nil, nil)
+		return nil, nil
 	})
 	exhausted := NewGenerator(empty, NewDict()).ToObject()
 	mustNotRaise(ListType.Call(newFrame(nil), Args{exhausted}, nil))
@@ -46,7 +46,7 @@ func TestGeneratorNext(t *testing.T) {
 
 func TestGeneratorSend(t *testing.T) {
 	empty := NewBlock(func(f *Frame, _ *Object) (*Object, *BaseException) {
-		return nil, f.Raise(StopIterationType.ToObject(), nil, nil)
+		return nil, nil
 	})
 	cases := []invokeTestCase{
 		invokeTestCase{args: wrapArgs(NewGenerator(empty, NewDict()), 123), wantExc: mustCreateException(TypeErrorType, "can't send non-None value to a just-started generator")},
@@ -78,7 +78,7 @@ func TestGeneratorSimple(t *testing.T) {
 		f.PushCheckpoint(2)
 		return NewStr("bar").ToObject(), nil
 	Yield2:
-		return nil, f.Raise(StopIterationType.ToObject(), nil, nil)
+		return nil, nil
 	})
 	cas := &invokeTestCase{
 		args: wrapArgs(NewGenerator(b, NewDict())),