interrupt_yield.py

class Ref:
    sentinel = object()
    def __init__(self):
        self._val = self.sentinel
    def set(self, val):
        self._val = val
    def get(self):
        t = self._val
        self._val = self.sentinel
        return t

def interruptible(ref, gen):
    for i in gen:
        x = ref.get()
        if x is not Ref.sentinel:
            yield x
        yield i

global_ref = Ref()

def foo():
  for res in interruptible(global_ref, bar()):
    print(res)
  return 0

def bar():
  yield 4
  yield 3
  baz()
  yield 1

def baz():
  # somehow force bar to `yield`
  # or `return` 2
    global_ref.set(2)

print(foo())