How can I use python's argparse with a predefined argument string?

I want to use the pythons argparse module to parse my cli parameter string. This works for the parameters a pass from terminal, but not with a given string.

import argparse

parser = argparse.ArgumentParser(description='Argparse Test script')
parser.add_argument("param", help='some parameter')

argString = 'someTestFile'
print(argString)

args = parser.parse_args(argString)

If I run this script I get this output:

~/someTestFile
usage: argparsetest.py [-h] param
argparsetest.py: error: unrecognized arguments: o m e T e s t F i l e

The ~/someTestFile is somehow transformed in o m e T e s t F i l e. As already mentioned, it works if I pass the filename from the terminal.

I could imagine, that this has something to do with string encodings. Does someone has an idea how to fix this?