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This may be trivial, but for me takes long time to solve in Python. I tried to implement multiprocessing and I used pool. I need to pass multiple arguments. Then I implement like 

def doWork(worker_args):
bbox_list, conf_list, all_rects, w, h, pix_per_w, n = worker_args

sys.stdout.write("w: %d%%   \n" % n )
sys.stdout.flush()

for k in range(h * w):
    conf = conf_list[n][k,1].flatten()[0]
    if conf > 0.75:
       y = int(k / w)
       x = int(k % w)
       bbox = bbox_list[n][k]
       abs_cx = pix_per_w/2 + pix_per_w*x + int(bbox[0,0,0])
       abs_cy = pix_per_h/2 + pix_per_h*y+int(bbox[1,0,0])
       w = bbox[2,0,0]
       h = bbox[3,0,0]
       all_rects[y][x].append(Rect(abs_cx,abs_cy,w,h,conf))

Then from main, I sent as

         all_rects = [[[] for x in range(net_config["grid_width"])] for y in range(net_config["grid_height"])]
         pool = multiprocessing.Pool(len(bbox_list))
         worker_args = [(bbox_list, conf_list, all_rects, net_config["grid_width"], net_config["grid_height"], pix_per_w, range(len(bbox_list)))]
         pool.map(doWork, worker_args)

Then error in printing n in doWork function is

TypeError: %d format: a number is required, not list

If I implement like

def doWork(n): and 
in main
pool.map(doWork, range(len(bbox_list)))

It works. Send data from 0 to 5.

Then I implement like

def doWork(worker_args): and 
in main
worker_args = [(bbox_list, conf_list, all_rects, net_config["grid_width"], net_config["grid_height"], pix_per_w)]
pool.map(doWork, worker_args)

It works also. But if I combine two as in the first example, then I got error at n. How to send n in single digit, not in list? Thanks

batuman
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  • Could the answers in [this question](http://stackoverflow.com/questions/5442910/) help? E.g., [one answer](http://stackoverflow.com/a/21292849/1016514) mentions a small library, *parmap* that extends the Python map to handle multiple arguments. – Hans Oct 21 '15 at 10:50
  • firstly, I don't think `map` from `multiprocessing` works as you're expecting. it will call you `doWork` function with each element of the `worker_args`. If you want to pass multiple parameters they need to be `zip`ped up, or an alternative could be to pass an `initializer` to the pool and pass in the shared state there. – Sam Mason Oct 21 '15 at 11:40
  • @SamMason, thanks. My another issue is the way I implement, it has memory issue. I am expecting send by reference. Actually it is not. How to implement send by reference? (http://stackoverflow.com/questions/986006/how-do-i-pass-a-variable-by-reference) this discussion says about byvalue or byreference. – batuman Oct 22 '15 at 05:12
  • This article (http://docs.python.org/3/faq/programming.html#how-do-i-write-a-function-with-output-parameters-call-by-reference) passed by assignment. Since assignment just creates references to objects, there’s no alias between an argument name in the caller and callee, why I have memory issue? All memories are used and program hanged after running sometime. – batuman Oct 22 '15 at 05:17
  • I'd post a separate question about variable references. Judging from your other questions you seem trying to learn a lot all at once, try slowing down and learning each concept in greater detail before trying to combine them with other new ideas – Sam Mason Oct 23 '15 at 19:34

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