I've now used Python's argparse module and I like it

March 30, 2016

The argparse module is the Python 2.7+ replacement for the old optparse module, which itself was basically an extension of the basic getopt module. A number of years ago, when I could barely start using argparse, I took a look at argparse's documentation and wound up with rather negative feelings about it. Having now written a program or two that uses argparse, I'm going to take those old views back.

I don't yet have any opinion on argparse as more than an optparse replacement for putting together quick argument handling for simple commands, but there are a number of things that I like about it for that role. In no particular order:

  • argparse doesn't produce warnings from pychecker. I know, this is a petty thing, but it's still nice to be able to just run 'pychecker program.py' without having to carefully guard 'import optparse' with various magic bits of code.

  • It's nice to be able to skip setting a default value for boolean flags with a store_true or store_false action. One less bit of code noise.

  • argparse gives you a simple way to define conflicting options. It isn't all that general but just having it there means that my programs have somewhat better option error checking. If I had to do it by hand, I might be tempted to not bother.

    (Because of the lack of generality, argparse doesn't give you top notch handling of conflicting arguments; if you want to do a really good job in even moderately complicated situations, you'll have to at least partially roll your own. But argparse is good enough for handling obvious cases in a simple program that you don't expect to be misused except by accident.)

  • It's conveniently lazy to let argparse handle positional arguments too. You can just tell it that there must be exactly N, or at least one, or whatever, and then continue onwards knowing that argparser will take care of all of the error checking and problem reporting and so on. If it gets to your code, you have at least the right number of arguments and you can pull them off the Namespace object it returns.

    (If you want to go a little bit crazy you can do a bunch of argument type validation as argparse processes the arguments. I'm not convinced that this is worth it for simple programs.)

The result of all of this is to reduce the amount of more or less boilerplate code that a simple argparse-using program needs to contain. Today I wrote one where the main function reduced down to:

def main():
   p = setup_args()
   opts = p.parse_args()
   for grname in opts.group:
      process(grname, opts)

All of the 'must have at least one positional argument' and 'some options obviously conflict' and so on error handling was entirely done for me in the depths of parse_args, so my code here didn't even have to think about it.

(I've historically shoved all of the argument parser setup off into a separate function. It's sufficiently verbose that I prefer to keep it out of the way of the actual logic in my main() function; otherwise it can be too hard to see the logic forest for the argument setup trees. With a separate setup_args() function, I can just skip over it entirely when reading the code later.)

Written on 30 March 2016.
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Last modified: Wed Mar 30 23:41:54 2016
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