Why unrecognized Argparse arg1 in bare "my_script .py arg1" vs "python my_script.py arg1"

I can run a script on the command line with an argument either as the bare "script.py arg1" or I can run it as "python script.py arg1". Both work until I use argparse for command line parameters.

The bare script does not recognize arg1:

Directories.py
usage: directories.py [-h] [-f FILE] directory
directories.py: error: the following arguments are required: directory

"python directories.py arg1" does work with argparse.

WHY?

Can I fix that bare script? Can I make the bare "directories.py arg1" work?

(I don't really understand why under Windows the bare script executes. I assume there is a python association.)

Here is the argparse part:

if __name__ == "__main__":
    
    parser = argparse.ArgumentParser(description="Get tree from single dir or dir list from file")  
    
    parser.add_argument("directory", type=str, help="Get single directory")
    
    # Adding argument
    parser.add_argument("-f", "--file", help = "Get directories from a file")
    
    # Read arguments from command line
    args = parser.parse_args()
        
    main(args)