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

Another option is to use shlex.split. It it especially very convenient if you have real CLI arguments string:

import shlex
argString = '-vvvv -c "yes" --foo bar --some_flag'
args = parser.parse_args(shlex.split(argString))

parser.parse_args() expects a sequence in the same form as sys.argv[1:]. If you treat a string like a sys.argv sequence, you get ['s', 'o', 'm', 'e', 'T', 'e', 's', 't', 'F', 'i', 'l', 'e']. 's' becomes the relevant argument, and then the rest of the string is unparseable.

Instead, you probably want to pass in parser.parse_args(['someTestFile'])

Just like the default sys.argv is a list, your arguments have to be a list as well.

args = parser.parse_args([argString])