Today, for reasons outside the scope of this entry, I updated one of my very old little Python utilities to add some features and move it to Python 3 in the process. This program was so old it was using getopt, so as part of updating it I switched it over to argparse, which is what I use in all of my modern programs. The change was positive in a variety of ways, but one of the things it did was it immediately caused me to add some more command line options. This isn't due to anything specific to this program, because over and over again I've had much the same experience; my argparse based programs have more options (and often better structured ones).

In thinking about it I believe there's a couple of reasons for this. First, argparse makes it easy to add the basic support for an option in a single place, in a ArgumentParser.add_argument() call. Unlike with getopt's much more primitive argument handling, I don't have to modify code in several places just to have my program accept the option, handle it to set or record some value, and include it in usage and help. Even if the option does nothing this is an easy first step.

Second, that argparse generates an 'args' (or 'options') object with all of the parsed information on it often makes it easy to expose new options to the code that needs to look at them. My usual pattern with argparse is to pass the 'opts' object you get from ArgumentParser.parse_args() down to functions that get called to do the work, rather than break out specific flags, options, and so on separately. This means that a new option is usually pervasively available through the code and doesn't have to be passed in specifically; I can just check 'opts.myNewOption' wherever I want to use it. By contrast, getopt didn't create a natural options object so I tended to pass around options separately (or worse, set global variables because it was the easy lazy way).

This doesn't always work; sometimes I need the new option in a place where I haven't already passed in the opts object. But it works a lot of the time, and when it works it makes it that much easier to code up the use of the new option.

(This also means it's easy to reverse out of an option I decide that I don't want after all. I can delete the creation of it and the use of it without having to go through changing function arguments around. Similarly it's easy to change how an option works because that too doesn't touch many places in the code.)