Claudiu Claudiu - 17 days ago 12
Python Question

python: windows equivalent of SIGALRM

I have this decorator:

def timed_out(timeout):
def decorate(f):
if not hasattr(signal, "SIGALRM"):
return f

def handler(signum, frame):
raise TimedOutExc()

@functools.wraps(f)
def new_f(*args, **kwargs):
old = signal.signal(signal.SIGALRM, handler)
signal.alarm(timeout)
try:
result = f(*args, **kwargs)
finally:
signal.signal(signal.SIGALRM, old)
signal.alarm(0)
return result

new_f.func_name = f.func_name
return new_f

return decorate


The code only does anything on linux, though, as on windows, there is no
SIGALRM
. What would be the simplest way to have this code work in Windows as well?

Answer

It's not very pretty, but I had to do something similar in a cross-platform way, and I came up with using a separate thread. Signal based systems did not work on all platforms reliably.

Use of this class could be wrapped up in a decorator, or made into a with context handler.

YMMV.

#!/usr/bin/env python2.7
import time, threading

class Ticker(threading.Thread):
  """A very simple thread that merely blocks for :attr:`interval` and sets a
  :class:`threading.Event` when the :attr:`interval` has elapsed. It then waits
  for the caller to unset this event before looping again.

  Example use::

    t = Ticker(1.0) # make a ticker
    t.start() # start the ticker in a new thread
    try:
      while t.evt.wait(): # hang out til the time has elapsed
        t.evt.clear() # tell the ticker to loop again
        print time.time(), "FIRING!"
    except:
      t.stop() # tell the thread to stop
      t.join() # wait til the thread actually dies

  """
  # SIGALRM based timing proved to be unreliable on various python installs,
  # so we use a simple thread that blocks on sleep and sets a threading.Event
  # when the timer expires, it does this forever.
  def __init__(self, interval):
    super(Ticker, self).__init__()
    self.interval = interval
    self.evt = threading.Event()
    self.evt.clear()
    self.should_run = threading.Event()
    self.should_run.set()

  def stop(self):
    """Stop the this thread. You probably want to call :meth:`join` immediately
    afterwards
    """
    self.should_run.clear()

  def consume(self):
    was_set = self.evt.is_set()
    if was_set:
      self.evt.clear()
    return was_set

  def run(self):
    """The internal main method of this thread. Block for :attr:`interval`
    seconds before setting :attr:`Ticker.evt`

    .. warning::
      Do not call this directly!  Instead call :meth:`start`.
    """
    while self.should_run.is_set():
      time.sleep(self.interval)
      self.evt.set()
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