Python - apply_async doesn't execute function

Question

Hi I'm trying to use multiprocessing to speed up my code. However, the apply_async doesn't work for me. I tried to do a simple example like:

from multiprocessing.pool import Pool
t = [0, 1, 2, 3, 4, 5]
def cube(x):
    t[x] = x**3
pool = Pool(processes=4)
for i in range(6):
    pool.apply_async(cube, args=(i, ))
for x in t:
    print(x)

It does not really change t as I would expect.

My real code is like:

from multiprocessing.pool import Pool
def func(a, b, c, d):
    #some calculations
    #save result to files
    #no return value
lt = #list of possible value of a
#set values to b, c, d
p = Pool()
for i in lt:
    p.apply_async(func, args=(i, b, c, d, ))

Where are the problems here?

Thank you!

Update: Thanks to the comments and answers, now I understand why my simple example won't work. However, I'm still in trouble with my real code. I have checked that my func does not rely on any global variable, so it seems not to be the same problem as my example code.

As suggested, I added a return value to my func, now my code is:

f = Flux("reactor")
d = Detector("Ge")
mv = arange(-6, 1.5, 0.5)
p = Pool()
lt = ["uee", "dee"]
for i in lt:
    re = p.apply_async(res, args=(i, d, f, mv, ))
    print(re.get())
p.close()
p.join()

Now I get the following error:

Traceback (most recent call last):
  File "/Users/Shu/Documents/Programming/Python/Research/debug.py", line 35, in 
print(re.get())
  File "/usr/local/Cellar/python3/3.6.0/Frameworks/Python.framework/Versions/3.6/lib/python3.6/multiprocessing/pool.py", line 608, in get
raise self._value
  File "/usr/local/Cellar/python3/3.6.0/Frameworks/Python.framework/Versions/3.6/lib/python3.6/multiprocessing/pool.py", line 385, in _handle_tasks
put(task)
  File "/usr/local/Cellar/python3/3.6.0/Frameworks/Python.framework/Versions/3.6/lib/python3.6/multiprocessing/connection.py", line 206, in send
self._send_bytes(_ForkingPickler.dumps(obj))
  File "/usr/local/Cellar/python3/3.6.0/Frameworks/Python.framework/Versions/3.6/lib/python3.6/multiprocessing/reduction.py", line 51, in dumps
cls(buf, protocol).dump(obj)
AttributeError: Can't pickle local object 'Flux.__init__..'

Python - apply_async doesn't execute function

2 Answers