I will use ls as an example:
I tried:
str = call("ls")
str stored 0.
I tried:
str = call("ls>>txt.txt")
but had no luck (an error occured & txt.txt wasn't created).
How do I perform this straight-forward task.
[I had imported call]
Edit: call simply runs a command in the shell, then returns the return value of that command.  Since your ls was successful, call('ls') returned 0.
If you are simply trying to run ls, then I would suggest using glob:
from glob import glob
file_list = glob('*')
If you want to do something more complicated, my suggestion would be to use subprocess.Popen() like this:
from subprocess import Popen, PIPE
ls_proc = Popen('ls', stdout=PIPE, stderr=PIPE)
out, err = ls_proc.communicate()
Note, in the stdout and stderr keywords, PIPE can be replaced with any file-like object, so you can send the output to a file if you want.  Also, you should read about Popen.communicate() prior to using it since if the command you call hangs, your python routine will also hang.
