set operation timers in python

timeout.py

from __future__ import division, with_statement
import signal

__all__ = ['TimeOutError', 'timeout']

class TimeOutError(Exception):
    def __init__(self):
        Exception.__init__(self,'Timeout')

def _raise_TimeOut(sig, stack):
    raise TimeOutError()

class timeout(object):
    def __init__(self, timeout, raise_exception=True):
        self.timeout = timeout
        self.raise_exception = raise_exception

    def __enter__(self):
        self.old_handler = signal.signal(signal.SIGALRM, _raise_TimeOut)
        signal.alarm(self.timeout)

    def __exit__(self, exc_type, exc_val, exc_tb):
        signal.signal(signal.SIGALRM, self.old_handler)
        signal.alarm(0)
        if exc_type is not TimeOutError:
            return False # Never swallow other exceptions
        return not self.raise_exception