clapy-derive 0.2.0

A clap-inspired declarative CLI argument parser for Python

clapy-derive

A clap-inspired declarative CLI argument parser for Python.

Define your CLI as a plain class with type annotations — clapy-derive handles parsing, help generation, and type conversion.

from clapy_derive import clapy_arg, clapy_parser

@clapy_parser("version", "verbose")
class Cli:
    """My awesome tool."""

    input: str = clapy_arg(short=True, long=True, help="input file")
    output: str = clapy_arg(short=True, long=True, default="output.txt")

if __name__ == "__main__":
    cli = Cli.parse()
    print(cli.input)
    print(cli.output)

$ python main.py --input data.csv
data.csv
output.txt

$ python main.py -i data.csv -o result.txt
data.csv
result.txt

$ python main.py --help
usage: cli [-h] [-V] [-v] -i INPUT [-o OUTPUT]

My awesome tool.

options:
  -h, --help            show this help message and exit
  -V, --version         show program's version number and exit
  -v, --verbose         Enable verbose output
  -i, --input INPUT     input file
  -o, --output OUTPUT

Installation

pip install clapy-derive

Usage

Positional arguments

A field with no clapy_arg() becomes a required positional argument — the same as clap's default behaviour for bare struct fields.

@clapy_parser()
class Cli:
    name: str          # positional, required
    greeting: str = "hello"  # positional, optional with default

$ python main.py alice

Named flags (--long / -s)

Use clapy_arg(short=True, long=True) to add -x / --xxx flags.

@clapy_parser()
class Cli:
    output: str = clapy_arg(short=True, long=True, default="out.txt")
    count:  int = clapy_arg(short=True, long=True, default=1)

• short=True — auto-assign the first character of the field name (-o, -c)
• long=True — use the field name as the long flag (--output, --count)
• Pass an explicit string to override: short="O", long="out"

Underscores in field names are converted to hyphens on the CLI (output_file → --output-file).

Optional values (T | None)

Annotate with T | None to make an argument optional. The value is None when not provided.

@clapy_parser()
class Cli:
    config: str | None = clapy_arg(short=True, long=True)

$ python main.py          # cli.config is None
$ python main.py -c x    # cli.config == "x"

Multiple values (list[T])

Annotate with list[T] to accept one or more values.

@clapy_parser()
class Cli:
    files: list[str] = clapy_arg(short=True, long=True)

$ python main.py --files a.txt b.txt c.txt

Boolean flags

A bool field becomes a store-true flag (default False).

@clapy_parser()
class Cli:
    debug: bool = clapy_arg(short=True, long=True)

$ python main.py --debug   # cli.debug is True
$ python main.py           # cli.debug is False

Built-in flags

Pass strings to @clapy_parser() to enable built-in flags:

String	Flags added
"version"	-V / --version (reads version from package metadata)
"verbose"	-v / --verbose (sets cli.verbose: bool)

@clapy_parser("version", "verbose")
class Cli:
    ...

Type conversion

clapy-derive infers the conversion type from the field annotation automatically.

Annotation	CLI input	Python value
str	"hello"	"hello"
int	"42"	42
float	"3.14"	3.14
pathlib.Path	"/tmp/f"	Path("/tmp/f")
bool	flag present	True

clapy_arg() reference

Parameter	Type	Default	Description
short	bool | str	False	-x flag. True = first char of field name
long	bool | str	True	--xxx flag. True = field name
help	str	""	Help text shown in --help
default	Any	(required)	Default value; makes the argument optional
choices	list | None	None	Restrict to a set of allowed values
metavar	str | None	None	Placeholder name in help output
env	str | None	None	Environment variable to fall back to

Requirements

• Python 3.12+

License

MIT