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【妙用协程】 - 可resume的异常处理

标准的异常处理是这样的

try:  print('hello')  raise Exception()  print('!!!') except:  print('world')  print('???')

这段代码会打印出???而不会打印出!!!，因为异常会中断当前流程，跳转到except部分去继续执行。但是有的时候我们希望的这样的行为：

try:  print('hello')  print(Scheduler.interrupt())  print('!!!') except ProcessInterrupt as pi:  pi.resume('world')  print('???')

这段代码打印出!!!而不是???，因为resume的时候把执行重新跳转回interrupt的地方了。这种行为类似vba里的on error resume next（ https://msdn.microsoft.com/en-us/library/5hsw66as.aspx ）。

如何实现的？其实原理上很简单。interrupt的时候把当前协程的状态保存起来（pickle.dumps），如果决定要resume，就把协程interrupt的时刻的状态重新恢复（pickle.loads）然后从那个点继续执行。

完整的代码（需要pypy或者stackless python）：

import greenlet import cPickle as pickle import traceback import threading import functools class ProcessInterrupt(Exception):  def __init__(self, interruption_point, pi_args):   self.interruption_point = interruption_point   self.stacktrace = traceback.extract_stack()   self.pi_args = pi_args  def resume(self, resume_with=None):   Scheduler.resume(self.interruption_point, resume_with)  def __repr__(self):   return '>>>ProcessInterrupt>>>%s' % repr(self.stacktrace)  def __str__(self):   return repr(self)  def __unicode__(self):   return repr(self) class Scheduler(object):  current = threading.local()  def __init__(self):   if getattr(self.current, 'instance', None):    raise Exception('can not have two scheduler in one thread')   self.scheduler_greenlet = greenlet.getcurrent()   self.current.instance = self  def __call__(self, action, action_args):   next = action, action_args   while next:    action, action_args = next    if 'init' == action:     next = action_args['init_greenlet'].switch()    elif 'interrupt' == action:     interruption_point = pickle.dumps(action_args['switched_from'])     should_resume, resume_with = False, None     next = action_args['switched_from'].switch(      should_resume, resume_with, interruption_point)    elif 'resume' == action:     should_resume, resume_with, interruption_point = True, action_args['resume_with'], action_args[      'interruption_point']     next = pickle.loads(action_args['interruption_point']).switch(      should_resume, resume_with, interruption_point)    else:     raise NotImplementedError('unknown action: %s' % action)  @classmethod  def start(cls, init_func, *args, **kwargs):   scheduler = Scheduler()   init_greenlet = greenlet.greenlet(functools.partial(init_func, *args, **kwargs))   scheduler('init', {    'init_greenlet': init_greenlet,   })  @classmethod  def interrupt(cls, pi_args=None):   should_resume, resume_with, interruption_point = cls.switch_to_scheduler('interrupt', {    'switched_from': greenlet.getcurrent()   })   if should_resume:    return resume_with   else:    pi = ProcessInterrupt(interruption_point, pi_args)    raise pi  @classmethod  def resume(cls, interruption_point, resume_with=None):   cls.switch_to_scheduler('resume', {    'interruption_point': interruption_point,    'resume_with': resume_with   })  @classmethod  def switch_to_scheduler(cls, *args, **kwargs):   return cls.current.instance.scheduler_greenlet.switch(*args, **kwargs) if '__main__' == __name__:  def init():   try:    print('hello')    print(Scheduler.interrupt())    print('!!!')   except ProcessInterrupt as pi:    pi.resume('world')    print('???')   try:    print('hello')    raise Exception()    print('!!!')   except:    print('world')    print('???')  Scheduler.start(init)

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