I have a python script which runs a particular script large number of times (for monte carlo purpose) and the way I have scripted it is that, I queue up the script the desired number of times it should be run then I spawn threads and each thread runs the script once and again when its done.

Once the script in a particular thread is finished, the output is written to a file by accessing a lock (so my guess was that only one thread accesses the lock at a given time). Once the lock is released by one thread, the next thread accesses it and adds its output to the previously written file and rewrites it. 

I am not facing a problem when the number of iterations is small like 10 or 20 but when its large like 50 or 150, python returns a KeyError: 51 telling me element doesn't exist and the error it points out to is within the lock which puzzles me since only one thread should access the lock at once and I do not expect an error. 

This is the class I use:

class errorclass(threading.Thread):
    
     def __init__(self, queue):
         self.__queue=queue
         threading.Thread.__init__(self)
     
     def run(self):
         while 1:
               item = self.__queue.get()
               if item is None: break
               result = myfunction()
               lock = threading.RLock()
               lock.acquire()
               ADD entries from current thread to entries in file and
               REWRITE FILE           
               lock.release()