argsl️

argsl is for everybody who finds argparse too verbose.

argsl (“args DSL” / “argparse DSL”) lets you define CLI arguments in a tiny DSL and get an argparse.Namespace back — with types, defaults, and optional --help text.

from argsl import argsl

args = argsl("""
filename <path>         # positional file/path
--name|-n <str!>        # required option
--debug|-d <flag>       # boolean switch
""")

…instead of writing argparse boilerplate.

---

• Expressive: one line per argument
• No boilerplate: returns argparse.Namespace (parsed from sys.argv)
• Typed: int, float, str, path, bool, flag, choice
• Compatible with argparse: wraps it internally

---

Installation

Using uv (recommended)

uv add argsl

Or with pip

pip install argsl

---

Basic Example

from argsl import argsl

args = argsl("""
filename <path>                          # positional file/path
--name|-n <str=env:USER>                 # optional with env fallback
--level|-l <choice:low,med,high="med"> # choice with default
--debug|-d <flag>                        # boolean flag
--onlylong <int=1>                       # long-only argument
--no-cache <flag>                        # store_true flag; app decides semantics
""")

print(vars(args))

In your pyproject.toml:

[project.scripts]
run = "main:main"

Run it:

uv run run file.txt --name Alice --debug --level high --no-cache

---

DSL Format

Each non-empty line describes one argument:

<flags or positional> <type-expression> [# help text]

Examples:

• Positional:
  • filename <path>
• Long option only:
  • --count <int=3>
• Long + short option:
  • --name|-n <str!>

Flags / option names

• Use --long for long options and -s for short options.
• Combine long and short options with | without spaces:
  • --name|-n <str!>
  • --name | -n <str!> (avoid spaces around |)

Inline help text with #

If a line contains #, everything after the first # becomes the help= text passed to argparse.

args = argsl("""
--debug|-d <flag>  # Enable debug logging
""")

Notes (current behavior):

• The split is done on the first # anywhere in the line.
• Help text is trimmed (strip()).
• Lines starting with # are ignored.
• # is not quote-aware. If you put # inside a quoted default (e.g. <str="a#b">), it will still be treated as the start of help text.

---

Supported Type Expressions

Type expressions live inside <...>.

Built-in types

DSL	Meaning	argparse behavior
<str>	string value	type=str
<int>	integer value	type=int
<float>	float value	type=float
<path>	filesystem path	type=pathlib.Path
<bool>	boolean value from a string	parses 1/true/yes (case-insensitive)
<flag>	boolean flag	action="store_true" (default False, becomes True if present)

Required (!)

• ! marks named options as required:
  • --name <str!> → required=True
• For positionals, argparse already requires them by default, so ! is effectively redundant.

Defaults (=)

You can provide defaults with =.

--count <int=3>
--title <str="hello world">

Defaults are parsed with ast.literal_eval when possible (numbers, quoted strings, etc.). If parsing fails, the raw string is used.

Environment fallback (=env:VAR)

Use an environment variable as a default:

--user <str=env:USER>
--home <path=env:HOME>

This sets default=os.getenv("VAR") (which may be None if the variable is unset).

Multiple values (*)

Add * to accept one or more values:

--list <str*>

This maps to nargs="+".

Choices (choice:)

Restrict input to a fixed set of strings:

--level <choice:low,med,high>

You can also set a default using =:

--level <choice:low,med,high="med">
--level <choice:low,med,high=med>

(Choices are always treated as strings.)

---

Supported Flag Variants

args = argsl("""
--user|-u <str!>      # short + long
--only <int=1>        # long-only option
--debug <flag>        # store_true
--no-cache <flag>     # store_true; app interprets as "disable cache"
""")

---

Test Coverage & Edge Cases

Covered in tests (see tests/test_argsl.py and tests/test_argsl_all.py):

• required positional + typed
• required named args (--foo <str!>)
• optional args with defaults (--bar <int=42>)
• multiple values (--list <str*>)
• boolean flags (--debug <flag>)
• env fallback (--x <str=env:HOME>)
• quoted defaults (--title <str="The Answer">)
• long-only and short+long (--onlylong, --msg|-m)
• choice types + defaults (--level <choice:low,med,high="med">)

---

Development Workflow

Using uv:

uv add .[dev]
uv add '.[dev]'   # on zsh (macos)
uv run test

Or with classic tools:

pip install -e .[dev]
pip install -e '.[dev]'   # on zsh (macos)
pytest

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Publishing to PyPI

rm -rf dist/  # remove old builds (PyPI rejects re-uploads)
uv build
uv publish

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MIT licensed • Built by Gwang-Jin Kim