I would like to pass a list of values and have each of them passed as an independent Thread:

For example:

import time
import threading

list_args = [arg1, arg2, arg3, ...., argn]

# Code to execute in an independent thread
import time
def countdown(n):
    while n > 0:
        print('T-minus', n)
        n -= 1
        time.sleep(0.5)


# Create and launch a thread
t1 = threading.Thread(target=countdown, args=(arg1,))
t2 = threading.Thread(target=countdown, args=(arg2,))
.
.
.
tn = threading.Thread(target=countdown, args=(argn,))
t1.start(); t2.start(); tn.start()

Quick fix

Call .join() on each of your t1, t2, etc. threads at the end.

Details

I suspect your real question is 'why isn't countdown being called?' The Thread.join() method causes the main thread to wait for other threads to finish executing before proceeding. Without it, once the main thread finishes, it terminates the whole process.

In your program when the main thread completes execution, the process is terminated along with all its threads, before the latter can call their countdown functions.

Other issues:

1. It's best to include a minimum working example. Your code cannot execute as written.

2. Normally some sort of data structure is used to manage threads. This is nice because it makes the code more compact, general, and reusable.

3. You needn't import time twice.

This might be close to what you want:

import time
import threading

list_args = [1,2,3]

def countdown(n):
    while n > 0:
        print('T-minus', n)
        n -= 1
        time.sleep(0.5)


# Create and launch a thread
threads = []
for arg in list_args:
    t = threading.Thread(target=countdown,args=(arg,))
    t.start()
    threads.append(t)

for thread in threads:
    thread.join()