FileConverter | File Converter is a very simple tool which allows you to convert and compress one or several file(s)

Finally, I found what was wrong! I took a deeper look in the logs file and specifically in the localhost*.log file and I found this:

A quote from the Python argparse docs:

Note that the object returned by parse_args() will only contain attributes for the main parser and the subparser that was selected by the command line (and not any other subparsers). So in the example above, when the a command is specified, only the foo and bar attributes are present, and when the b command is specified, only the foo and baz attributes are present.

This is exactly your case : You are not invoking the programs subcommand fileconcatenator, so the args object will not contain the arguments of that subcommand, e.g. autoconcat. You have to check first which subcommand was called. This can be done by having an option common for all subcommands, which is not modifiable by the command-line user. It will be set for each subcommand separately, and when subcommand a is called, this argument will have value a, and when subcommand b is called the argument will have value b. This can be achieve by calling set_defaults on each subparser like this: