Command Line Parsing in Python: Tips & Tricks

2012-12-13 21:49

Reading program’s command line and doing something with the arguments is the main purpose of most small (or bigger) utilities. Those are often written in Python – because of how easy and fast this is – so there should be a way to parse the command line in Python, too.
And in fact there are quite a few of them, all from the standard library. But the argparse module is most likely the best of them all, equally for its flexibility and power, as well as the sole fact of not being deprecated yet ;-)

For that matter, I have already used it several times, not only in Python. Today I want to present a summary of few useful techniques and solutions that I learned along the way, mostly by braving the not-so-friendly documentation of argparse. Given I’m not likely to do unusual stuff here, they should also address quite common, albeit less trivial use cases.

Boolean flags

Following the convention of every operating system imaginable, argparse has positional arguments and flags. Flags are denoted by one or two dashes preceding the name or its one-letter abbreviation:

  1. $ git commit -m "Fix stuff"
  2. $ hg bisect --bad 42
  3. $ ln -s ~/node_modules/foobar-0.0.1/bin/foobar ~/bin/foobar

Normally in argparse, flags take arguments that are later stored in the result object. This would be helpful for parsing something like the -m (message) flag in the git commit example above.
Not every flag needs to behave like that, though. In the last ln example, the -s does not take any arguments. Instead, it alters the program behavior by its mere presence: with it, ln creates a symbolic link instead of “hard” link. So in a sense, the flag is boolean. We would like to handle it as such.

In argparse, this is possible by setting the appropriate action= in the add_argument method:

  1. parser.add_argument("--symbolic", "-s", action='store_true', default=False)

Depending on what’s more logical for your program, you can reverse the logic to 'store_false' and default=True, of course.

Multiple positional arguments

If your program takes one entity as an argument and does something specific with it, users will often expect it to work with multiple entities too. You can observe it first hand with pip:

  1. $ pip install Flask
  2. $ pip install Flask WTForms SQLAlchemy celery pytz

or any version control application:

  1. $ git add README
  2. $ git add foo.h foo.c Makefile

There is no reason to ignore this expectation and it’s pretty easy to satisfy in argparse. Again, there is an action= for that:

  1. parser.add_argument("--foo", action='append')

and it’s sufficient for flags. Here the object returned by parse_args will get foo attribute with the list of arguments from all occurrences of --foo.

For positionals, it’s a little bit trickier because by default, they are meant to appear exactly once. This can be changed using nargs=:

  1. parser.add_argument("files", nargs='+')

The value of '+' is probably the most useful here, as it requires for the argument to be present at least once. Just like for flags, the result will be a list of all its occurrences, so you can iterate or map over it easily.

Optional positional arguments

Less typically, you may want to have a positional argument which can be supplied or not (an optional one). Although it is possible with the API outlined above, I wouldn’t recommend it: you will have to deal with unnecessary 0-or-1-element list and you won’t get proper error checking at the argparse level.

The correct solution involves nargs=, too, but with a dedicated '?' value:

  1. parser.add_argument("cache_dir", nargs='?', default='/tmp')

As you may guess, default= allows you to specify the value in parse_args result should the argument be omitted.


Once you set up your ArgumentParser, you will (hopefully) want to test it. Lucky for you, this can be done easily without every touching the actual command line. Simply pass your arguments (as a list) to parse_args and it will use it instead of sys.argv:

  1. >>> parser.parse_args(['-foo', 'bar'])
  2. Namespace(foo='bar')

With this you can easily write some nice unit tests for your parser – which you should do, obviously. What you should not do, however, is abusing this feature to call your program’s code from itself:

  1. def main(argv=sys.argv):
  2.     args = parse.parse_args(argv)
  3.     # ...
  5. # later, somewhere deep inside...
  6. main(['other_function', 'one_argument', '--key', 'value'])

Just don’t.

Read more

There are, of course, many other interesting features and applications of argparse that you will find useful. I can especially recommend that you get to know about:

  • subparsers, a way to divide your complex tool into several internal commands (like git or pip)
  • argument groups for organizing your arguments into functional groups for better --help output, or for mutual exclusion (e.g. --verbose and --quiet option)
  • help text formatting, handy for more elaborate descriptions that need their whitespace preserved

Equipped with this knowledge, you should be able to write beautiful and easy to use command line tools. Please do so :)

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Author: Xion, posted under Programming »

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