Confused regarding python positional arguments, something like args=(i,)

Question

Let's say I tested the following code to see the sub process pool act:

# coding=utf-8
import os
import sys
from multiprocessing import Pool
import time
import random
def run_proc(param1):
    print("child procees %s pid is %s,parent id is %s" %
          (param1, os.getpid(), os.getppid()))
    starttime = time.time()
    time.sleep(random.random() * 3)
    endtime = time.time()
    print('child process %s runs %0.2f seconds.' %
          (param1, (endtime - starttime)))

if __name__ == '__main__':
    print(sys.version)
    pname = sys.argv[0].split('/')[-1]
    print("process %s is running now...,it's pid is %s" % (pname, os.getpid()))
    p = Pool(5)
    for i in range(5):
        p.apply_async(run_proc, args=("test"+str(i),))  
    print("waiting for all subprocess to end...")
    p.close()
    p.join()
    print("all subprocesses are over!")

And that the output was all that I expected:

    3.5.0 (default, Jul 23 2017, 10:55:33) 
    [GCC 4.2.1 Compatible Apple LLVM 8.1.0 (clang-802.0.42)]
    process mp_basic_pool.py is running now...,it's pid is 19352
    waiting for all subprocess to end...
    child procees test0 pid is 19367,parent id is 19352
    child procees test1 pid is 19368,parent id is 19352
    child procees test2 pid is 19369,parent id is 19352
    child procees test3 pid is 19370,parent id is 19352
    child procees test4 pid is 19371,parent id is 19352
    child process test2 runs 0.93 seconds.
    child process test4 runs 1.33 seconds.
    child process test3 runs 1.68 seconds.
    child process test0 runs 2.68 seconds.
    child process test1 runs 2.90 seconds.
    all subprocesses are over!
    [Finished in 3.2s]

There is the line "p.apply_async(run_proc, args=("test"+str(i),))". When I first wrote this code, I wrote it as: "p.apply_async(run_proc, args=("test"+str(i)))". A comma left, but the output was as:

3.5.0 (default, Jul 23 2017, 10:55:33) 
[GCC 4.2.1 Compatible Apple LLVM 8.1.0 (clang-802.0.42)]
process mp_basic_pool.py is running now...,it's pid is 19382
waiting for all subprocess to end...
all subprocesses are over!
[Finished in 0.4s]

I looked for the python document and found that the second parameter should be a tuple, but is the comma be needed?

Yes, actually you read it correctly. You need a tuple, and just `(something)` is not a tuple, it's just `something`. In the other case `(something,)` is a tuple only composed by one element : `something`. — iFlo, Aug 04 '17 at 15:17
ok, I get confirmed now ,3Q, just like @Carcigenicate said, it's ambiguous for the interpreter — XpreZ, Aug 04 '17 at 15:25

Carcigenicate · Answer 1 · 2017-08-04T15:28:34.160

1

Single element tuples (which ("test"+str(i),) is) require a trailing comma to differentiate them from just a pair of parenthesis.

Think about it this way: for (x), without the comma, how is the interpreter supposed to know if you meant to use parenthesis for grouping, or to make a tuple? It's ambiguous.

edited Aug 04 '17 at 15:28

answered Aug 04 '17 at 15:08

Carcigenicate

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Confused regarding python positional arguments, something like args=(i,)

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