This will make boo more friendly to coroutine lovers. An yield statement without an expression will be equivalent to 'yield <zero value for the inferred domain>', in other words: def foo(): yield 1 yield # equivalent to yield 0 yield 3 def bar(): yield "foo" yield # equivalent to yield null When there is no inferred value (only yields without expression arguments) all yields fallback to 'yield null' and the type of the generator item is inferred to be 'object'.