system_call.py

# Excerpt from http://www.dabeaz.com/coroutines/pyos4.py
    
# coroutine that will be wrapped in a task
def foo():
    mytid = yield GetTid()
    ...

# A system call
class GetTid(SystemCall):
    def handle(self):
        # The scheduler "injects" task in the 
        # system call
        self.task.sendval = self.task.tid

class Scheduler(object):
    ...
    def mainloop(self):
       ...
       # RHS of foo's yied 
       result = task.run()
       if isinstance(result, SystemCall):
           result.task  = task
           result.sched = self
           result.handle()