Code to demonstrate issues of using iterators and generators in multi-threaded applications

count.py

import threading

def count():
    i = 0
    while True:
        i += 1
        yield i

class Counter:
    def __init__(self):
        self.i = 0

    def __iter__(self):
        return self

    def next(self):
        self.i += 1
        return self.i

def loop(func, n):
    for i in range(n):
        func()
        
def run(counter, repeats=1000, nthreads=10):
    threads = [threading.Thread(target=loop, args=(counter.next, repeats)) for i in range(nthreads)]

    for t in threads:
        t.start()

    for t in threads:
        t.join()
    

def main():
    c1 = count()
    c2 = Counter()

    run(c1, repeats=100000, nthreads=2)
    print "c1", c1.next()

    run(c2, repeats=100000, nthreads=2)
    print "c2", c2.next()

if __name__ == "__main__":
    main()

output

Exception in thread Thread-2:
Traceback (most recent call last):
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/threading.py", line 522, in __bootstrap_inner
    self.run()
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/threading.py", line 477, in run
    self.__target(*self.__args, **self.__kwargs)
  File "count.py", line 22, in loop
    func()
ValueError: generator already executing

c1 112106
c2 158368

threadsafe_counter.py

class Counter:
    def __init__(self):
        self.i = 0
        self.lock = threading.Lock()

    def __iter__(self):
        return self

    def next(self):
        with self.lock:
            self.i += 1
            return self.i

threadsafe_iter.py

class threadsafe_iter:
    def __init__(self, it):
        self.it = it
        self.lock = threading.Lock()

    def __iter__(self):
        return self

    def next(self):
        with self.lock:
            return self.it.next()