Command-line arguments in Python show up in `sys.argv` as a list of strings (so you'll need to import the `sys` module).

For example, if you want to print all passed command-line arguments:

    import sys
    print(sys.argv)

Command-line options are sometimes passed by position (e.g. `myprogram foo bar`) and sometimes by using a "-name value" pair (e.g. `myprogram -a foo -b bar`).

Here's a simple way to parse command-line pair arguments. It scans the `argv` list looking for `-optionname optionvalue` word pairs and places them in a dictionary for easy retrieval. The code is heavily commented to help Python newcomers.

    """Collect command-line options in a dictionary"""

    def getopts(argv):
        opts = {}  # Empty dictionary.
        while argv:  # While there are arguments left to parse.
            if argv[0][0] == '-':  # Found a "-name value" pair.
                opts[argv[0]] = argv[1]  # Add key and value to the dictionary.
                argv = argv[1:]  # Reduce the argument list.
            else:
                argv = argv[1:]
        return opts

    if __name__ == '__main__':
        from sys import argv
        myargs = getopts(argv)
        if '-i' in myargs:  # Example usage.
            print(myargs['-i'])
        print(myargs)

Running this script (note the first argument is always the script filename):

    $ python main.py -i input.txt -o output.txt
    input.txt
    {'-o': 'output.txt', '-i': 'input.txt'}

Simple solution, but not very robust; it doesn't handle error checking and the like. So don't use this in production code! There are more complex alternatives available. Some modules to consider are `getopt`, `optparse` (deprecated), and `argparse`.

This post was inspired by the book "Programming Python" by Mark Lutz.