Why does this use of Python's multiprocessing module not seem to process sequentially?

Question

I'm trying to learn to use Python's Multiprocessing module. As a first test, I thought I would run four 15 second processes at the same time. I wrote this module, which I called "multiPtest.py"::

import time
import timeit
import multiprocessing


def sleepyMe(napTime):
    time.sleep(napTime)
    print "Slept %d secs" % napTime

def tester(numTests):
    #Launch 'numTests' processes using multiProcessing module
    for _ in range(numTests):
        p = multiprocessing.Process(target=sleepyMe(15))
        p.start() #Launch an 'independent' process
        #p.join() ##Results identical with or without join

def multiTester():
    #Time running of 4 processes
    totTime = timeit.Timer('tester(4)', setup = 'from multiPtest import     tester').repeat(1,1)
    print "Total = ", totTime[0]

However, when I run, I get these results:

Python 2.7.6 (default, Mar 22 2014, 22:59:56) 
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from multiPtest import *
>>> multiTester()
Slept 15 secs
Slept 15 secs
Slept 15 secs
Slept 15 secs
Total =  60.0739970207

I would have expected the Total time to be closer to 15 seconds, rather than 60. I know that I have 4 cores, because I looked at /proc/cpuinfo:

~/Projects/PythonProjects$ cat /proc/cpuinfo 
processor   : 0
vendor_id   : GenuineIntel
cpu family  : 6 
model       : 60
model name  : Intel(R) Core(TM) i7-4900MQ CPU @ 2.80GHz
stepping    : 3
microcode   : 0x17
cpu MHz     : 800.000
cache size  : 8192 KB
physical id : 0
siblings    : 8
core id     : 0
cpu cores   : 4
...

Why don't I see these 4 sleepers asleep at the same time? Shouldn't I have been able to create and launch new processes while others were asleep/busy? Am I misunderstanding something about multiprocessing, about Python's multiprocessing module or something else?

Answer 1

In the line

p = multiprocessing.Process(target=sleepyMe(15))

you actually already call sleepyMe and use the result (None) as the value for the target parameter, thus waiting 15 seconds. Try

p = multiprocessing.Process(target=sleepyMe, args=(15, ))

and modify the function to join() all subprocesses after the for loop, or it will return immediately and you'll end up with a total time close to zero..