task_with_stack.py

class Task(object):
    def run(self):
        while True:
            try:
                result = self.target.send(self.sendval)
                
                if isinstance(result, SystemCall): 
                    return result
                
                # sub-routine call
                if isinstance(result, types.GeneratorType):
                    # put the caller on the stack
                    self.stack.append(self.target)
                    self.sendval = None
                    self.target  = result
                else:
                    if not self.stack: 
                        return
                    self.sendval = result
                    # resume execution to caller
                    self.target = self.stack.pop()
            # current coroutine finished
            except StopIteration:
                # invalid state 
                if not self.stack: 
                    raise
                self.sendval = None
                self.target = self.stack.pop()
                
# Example coroutine and sub-coroutine
def Accept(sock):
    yield ReadWait(sock)
    yield sock.accept()

def server(port):
    # ...
    while True:
        # coroutine that calls another
        client, addr = yield Accept(sock)
        yield NewTask(handle_client(client, addr))