Command-Line Arguments

Command-line programs receive their settings as arguments. The argparse module in the standard library reads those arguments, converts them to the types the program expects, and reports an error when they are missing or malformed. It is imported the same way as any other standard library module.

ArgumentParser holds the definition of the interface. Each call to add_argument declares one option: its name, the type its value is converted to, and the value used when the option is absent.

Reading the arguments

parse_args returns an object whose attributes hold the parsed values. A leading -- is stripped and internal hyphens become underscores, so --min-depth is read back as args.min_depth.

Because type=int was declared, the string "30" arrives as the integer 30. Without it, every value would remain a string.

Omitted options take their declared defaults.

Positional arguments and flags

A name declared without a leading -- is positional: it is required, and it is read from its place in the argument list. action="store_true" declares a flag that takes no value and is True only when present.

Where the arguments come from

Called with no argument, parse_args() reads sys.argv[1:], which is what the shell passed to the program. Called with a list, it parses that list instead. Doing the parsing inside a function that accepts the list makes the function testable without running the program from a shell.

When an argument is missing or cannot be converted, argparse prints a usage message and exits rather than raising an exception the program could catch. That behaviour is correct for a command-line program, and it is the reason no validation code for these options is written by hand.

Task

Write parse_options(argv). Build an ArgumentParser, declare --min-depth as an integer with default 10 and --gene with default "any", parse the given list, and return "depth=<min-depth> gene=<gene>".