Hey Magicans and Pythonistas!
I’m sharing ArgMojo, a command-line argument parser for Mojo, inspired by Python’s argparse, Rust’s clap, and Go’s cobra.
If you’ve ever used Python’s argparse, Rust’s clap, or Go’s cobra, you know how essential a good arg parser is for any CLI tool. I’m working on a CLI-based Chinese character search engine and a CLI calculator for Decimo, so I needed one for Mojo — and that’s how ArgMojo was born.
Demo: Shell tab-completion powered by ArgMojo
Installation
Available on modular-community:
pixi add argmojo
Available on Github release page (argmojo.mojopkg)
What it does
ArgMojo provides a builder-pattern API for defining and parsing CLI arguments. Core features include:
Parsing — long/short options, boolean flags, positional args, defaults, required args, -- stop marker, short flag merging (-abc), attached short values (-ofile), prefix matching (--verb → --verbose)
Value handling — choices validation, count flags (-vvv), negatable flags (--color / --no-color), append/collect (--tag x --tag y), value delimiter (--env a,b,c), multi-value nargs, key-value maps, numeric range validation
Groups — mutually exclusive, required-together, one-required, conditional requirements
Subcommands — hierarchical commands with nesting (e.g., git remote add), persistent (global) flags, auto-registered help subcommand
Help & UX — auto-generated --help with pixi-style ANSI colours, --version, metavar, hidden args, aliases, deprecated args, custom tips
Interactive — .prompt() to ask for missing values, .password() for masked input, confirmation_option() for --yes/-y skip-confirmation
Shell completion — generate_completion("bash"|"zsh"|"fish") produces tab-completion scripts
CJK support — CJK-aware 2-column-wide help alignment, full-width auto-correction (－－ｖｅｒｂｏｓｅ → --verbose), CJK punctuation detection (——verbose → --verbose)
Advanced — argument parents (shared definitions across commands), default-if-no-value / require-equals, remainder positional, partial parsing (parse_known_arguments()), allow hyphen values (stdin - convention), response files (@args.txt)
Validation — compile-time StringLiteral parameters validated at mojo build time; registration-time validation catches group constraint typos when the program starts
Demo
The repo ships with two complete example CLIs — a simulated grep (single command, showcasing all value/group features) and a simulated git (12 subcommands with nesting) — so you can see everything in action.
Here’s what ./mgit clone looks like:
The design is inspired by argparse/clap/cobra, but adapted to fit Mojo’s current capabilities. It is just structs and builder methods. I did a cross-language survey of some major CLI libraries, so the feature set is deliberate. The builder API is now fully parameterised with compile-time StringLiteral parameters.
Your feedback are very welcome. Just raise an issue if you find any bug or have any suggestion. Thanks for very much 