Python 2.6 with a Python 3.2-style semaphore

sem_test.py

import threading
import weakref
import time
import thread_util_copy

nthreads = 1000

sem = thread_util_copy.BoundedSemaphore(5)
local = threading.local()


class Vigil(object):
    pass


class MyThread(threading.Thread):
    def run(self):
        def on_thread_died(ref):
            sem.release()

        local.vigil = Vigil()
        self.ref = weakref.ref(local.vigil, on_thread_died)
        sem.acquire()


threads = [MyThread() for _ in range(nthreads)]
for t in threads:
    t.start()

for t in threads:
    t.join()

for _ in range(5):
    if sem.counter == 5:
        print 'success!'
        break
    getattr(local, 'c', None)  # Trigger cleanup in 2.6
    time.sleep(.1)

if sem.counter != 5:
    print 'fail', sem.counter

thread_util_copy.py

import threading

try:
    from time import monotonic as _time
except ImportError:
    from time import time as _time

### Begin backport from CPython 3.2 for timeout support for Semaphore.acquire
class Semaphore:

    # After Tim Peters' semaphore class, but not quite the same (no maximum)

    def __init__(self, value=1):
        if value < 0:
            raise ValueError("semaphore initial value must be >= 0")
        self._cond = threading.Condition(threading.Lock())
        self._value = value

    def acquire(self, blocking=True, timeout=None):
        if not blocking and timeout is not None:
            raise ValueError("can't specify timeout for non-blocking acquire")
        rc = False
        endtime = None
        self._cond.acquire()
        while self._value == 0:
            if not blocking:
                break
            if timeout is not None:
                if endtime is None:
                    endtime = _time() + timeout
                else:
                    timeout = endtime - _time()
                    if timeout <= 0:
                        break
            self._cond.wait(timeout)
        else:
            self._value = self._value - 1
            rc = True
        self._cond.release()
        return rc

    __enter__ = acquire

    def release(self):
        self._cond.acquire()
        self._value = self._value + 1
        self._cond.notify()
        self._cond.release()

    def __exit__(self, t, v, tb):
        self.release()

    @property
    def counter(self):
        return self._value


class BoundedSemaphore(Semaphore):
    """Semaphore that checks that # releases is <= # acquires"""
    def __init__(self, value=1):
        Semaphore.__init__(self, value)
        self._initial_value = value

    def release(self):
        if self._value >= self._initial_value:
            raise ValueError("Semaphore released too many times")
        return Semaphore.release(self)
### End backport from CPython 3.2