examples/os/os_system.py
from __future__ import print_function
import os

cmd = 'ls'
param = '-l'

command = cmd + ' ' + param
print(command)

os.system(command)

But of course if you wanted to list the content of a directory in an os independent way you'd use os.listdir('.') or you could use the glob.glob("*.py") function to have a subset of files.