Argument Parsing for Julia
Author kmsquire
5 Stars
Updated Last
3 Years Ago
Started In
January 2020


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This package is not registered yet, but can be installed directly from this repo. From the Julia REPL or in a Jupyter cell, run

]activate .  # Optionally activate a project

Or, equivalently, from the command line with

julia --project=. -e 'import Pkg; Pkg.add("")'

(Remove --project=. if not working in a project... but you should be!)


This is (another) argument parser for Julia, inspired by and based on Python's argparse library.

At the time of this writing, there are two other argument parsers available.

  1. ArgParse.jl, also largely based on Python's argparse module.
  2. DocOpt.jl, based on a the Python docopt module.

Both are actually quite feature full. What I found was that the startup, compile, and sometimes parse overhead were quite high for a project I was working on. ArgParse2.jl was an attempt to do better.

Some differences from ArgParse.jl:

  • More closely copies the interface from Python's argparse. In particular, all but one of the (supported) arguments to ArgumentParser and add_argument! have the same names. (const is a keyword in Julia, so that was renamed to constant)
  • ArgParse2.jl uses functions instead of macros for setting up the parser, so the parsing setup is somewhat different. Again, this is more similar to Python's argparse.
  • ArgParse2.jl returns a NamedTuple object with the parsed arguments; ArgParse.jl returns a dictionary. (NamedTuples did not exist in the language when ArgParse.jl was written, but it's output could trivially be put into one.)

DocOpt.jl is quite different (and quite cool). From my limited exploration, it doesn't quite give the same level of control over argument behavior, but has the advantage of a much more concise syntax.


using ArgParse2

in_danger() = rand(Bool)

function julia_main()::Cint
    parser = ArgumentParser(prog = "frodo",
                            description = "Welcome to Middle Earth",
                            epilog = "There is no real going back")

    add_argument!(parser, "surname", help = "Your surname")
    add_argument!(parser, "-s", "--ring-size", type = Int, help = "Ring size")
        action = "store_true",
        default = false,
        help = "Turn invisible when needed")
    add_argument!(parser, "--friends", metavar="FRIEND", nargs = "+", required = true)

    args = parse_args(parser)

    println("Welcome, Frodo $(args.surname)!")
    if args.ring_size === nothing
        println("Let's get you fitted for a ring.  We'll need to measure your ring size.")
        println("I see your ring size is $(args.ring_size)")

    if in_danger() && args.auto_hide
        println("Orcs are near!  You turn invisible.")

    if length(args.friends) > 0
        travel_companions = join(args.friends, ", ", " and ")
        println("You're traveling to Mordor with $travel_companions.")

    return 0


Running this gives

$ julia --project=.. frodo.jl -h
Usage: frodo.jl [-h] [-s RING_SIZE] [--auto-hide] --friends FRIEND
              [FRIEND ...] surname

Welcome to Middle Earth

Positional arguments:
 surname      Your surname

Optional arguments:
 -h, --help   show this help message and exit
 -s RING_SIZE, --ring-size RING_SIZE
              Ring size
 --auto-hide  Turn invisible when needed
 --friends FRIEND [FRIEND ...]
              friends help

There is no real going back
$ julia --project=.. frodo.jl Baggins --auto-hide --friends "Samwise Gangee" "Peregrin Took" "Meriadoc Brandybuck" "Fredegar Bolger"
Welcome, Frodo Baggins!
Let's get you fitted for a ring.  We'll need to measure your ring size.
Orcs are near!  You turn invisible.
You're traveling to Mordor with Samwise Gangee, Peregrin Took, Meriadoc Brandybuck and Fredegar Bolger.


At this point, ArgParse2.jl isn't feature full (e.g., commands are not implemented) but it is quite usable for simple command line parsing.

Load/compile time is currently faster than both parsers mentioned above, and (from some very minor testing) parse time is faster than ArgParse.jl and about on par with DocOpt.jl (for the features supported). That will probably change as more features are added (it might get slower). There's probably room for optimization as well.

There's no documentation yet--for now, you can use the example above (also in the examples directory) and the tests as guidelines.

If you're familiar with Python's argparse library, this library should feel very familiar.