Sharing a variable between processes

Question

I have a downloader function that downloads multiple files parallely. I use multiprocessing.Pool.map_async in order to download different chunks of the same file. I would like to show a statusbar of the download. For this, I need to know the total bytes that has been already downloaded (total_bytes_dl).

    pool = multiprocessing.Pool(processes)
    mapObj = pool.map_async(f, args)

    while not mapObj.ready():
        status = r"%.2f MB / %.2f MB" % (total_bytes_dl / 1024.0 / 1024.0, filesize / 1024.0 / 1024.0,)
        status = status + chr(8)*(len(status)+1)
        print status,
        time.sleep(0.5)

Is there a way to set a variable that will be shared among all these processes AND the main process, so every process can append the amount of bytes that has just downloaded?

Answer 1

The solution was to intilize the new process and pass the shared ctypes value:

from ctypes import c_int
import dummy

shared_bytes_var = multiprocessing.Value(c_int)

def Func(...):
    ....
    pool = multiprocessing.Pool(initializer=_initProcess,initargs=(shared_bytes_var,))
    ....

def _initProcess(x):
  dummy.shared_bytes_var = x