arcparse 0.3.0

Declare program arguments declaratively and type-safely

Arcparse

Declare program arguments declaratively and type-safely. Optionally set argument defaults dynamically (see Dynamic argument defaults).

This project provides a wrapper around argparse. It adds type-safety and allows for more expressive argument parser definitions.

Disclaimer: This library is young and probably highly unstable. Use at your own risk. Pull requests are welcome.

Example usage

from arcparse import ArcParser, positional


class Args(ArcParser):
    name: str = positional()
    age: int = positional()
    hobbies: list[str] = positional()
    happy: bool


args = Args.parse()
print(f"Hi, my name is {args.name}!")

For a complete overview of features see Features.

Installation

# Using pip
$ pip install arcparse

# locally using poetry
$ poetry install

Features

Required and optional arguments

Arguments without explicitly assigned argument class are implicitly options (prefixed with --). A non-optional typehint results in required=True for options. Defaults can be set by directly assigning them. You can use option() to further customize the argument.

class Args(ArcParser):
    required: str
    optional: str | None
    default: str = "foo"
    default_with_help: str = option(default="bar", help="help message")

Positional arguments

Positional arguments use positional(). Optional type-hints use nargs="?" in the background.

class Args(ArcParser):
    required: str = positional()
    optional: str | None = positional()

Flags

All arguments type-hinted as bool are flags, they use action="store_true" in the background. Use no_flag() to easily create a --no-... flag with action="store_false". Flags as well as options can also define short forms for each argument. They can also disable the long form with short_only=True.

class Args(ArcParser):
    sync: bool
    recurse: bool = no_flag(help="Do not recurse")

    debug: bool = flag("-d")  # both -d and --debug
    verbose: bool = flag("-v", short_only=True)  # only -v

Multiple values per argument

By type-hinting the argument as list[...], the argument will use nargs="*" in the background. Passing append=True to option() uses action="append" instead (this is available only for option()).

class Args(ArcParser):
    option_nargs: list[str]
    positional_nargs: list[str] = positional()

Type conversions

Automatic type conversions are supported. The type-hint is used in type=... in the background (unless it's str, which does no conversion). Using a StrEnum instance as a type-hint automatically populates choices. A custom type-converter can be used by passing converter=... to either option() or positional(). Come common utility converters are defined in converters.py.

Custom converters may be used in combination with multiple values per argument. These converters are called itemwise and need to be wrapped in itemwise(). This wrapper is used automatically if an argument is typed as list[...] and no converter is set.

from arcparse.converters import csv, itemwise

class Args(ArcParser):
    class Result(StrEnum):
        PASS = "pass"
        FAIL = "fail"

        @classmethod
        def from_int(cls, arg: str) -> Result:
            number = int(arg)
            return cls.PASS if number == 1 else cls.FAIL

    number: int
    result: Result
    custom: Result = option(converter=Result.from_int)
    ints: list[int] = option(converter=csv(int))
    results: list[Result] = option(converter=itemwise(Result.from_int))

Name overriding

Type-hinting an option as list[...] uses action="append" in the background. Use this in combination with name_override=... to get rid of the ...s suffixes.

class Args(ArcParser):
    values: list[str] = option(name_override="value")

Subparsers

Type-hinting an argument as a union of ArcParser subclasses creates subparsers in the background. Assigning from subparsers() gives them names as they will be entered from the command-line. Subparsers are required by default. Adding None to the union makes the subparsers optional.

class FooArgs(ArcParser):
    arg1: str

class BarArgs(ArcParser):
    arg2: int = positional()

class Args(ArcParser):
    action: FooArgs | BarArgs = subparsers("foo", "bar")

class OptionalSubparsersArgs(ArcParser):
    action: FooArgs | BarArgs | None = subparsers("foo", "bar")

Once the arguments are parsed, the different subparsers can be triggered and distinguished like so:

python3 script.py foo --arg1 baz
python3 script.py bar --arg2 123

args = Args.parse()
if isinstance(foo := args.action, FooArgs):
    print(f"foo {foo.arg1}")
elif isinstance(bar := args.action, BarArgs):
    print(f"bar {bar.arg2}")

Be aware that even though the isinstance() check passes, the instantiated subparser objects are never actual instances of their class because a dynamically created dataclass is used instead. The isinstance() relation is faked using a metaclass overriding __instancecheck__().

Dynamic argument defaults

The parse() classmethod supports an optional dictionary of defaults, which replace the statically defined defaults before parsing arguments. This might be useful for saving some arguments in a config file allowing the user to provide only the ones that are not present in the config.

Credits

This project was inspired by swansonk14/typed-argument-parser.

Known issues

Annotations

from __future__ import annotations makes all annotations strings at runtime. This library relies on class variable annotations's types being actual types. inspect.get_annotations(obj, eval_str=True) is used to evaluate string annotations to types in order to assign converters. If an argument is annotated with a non-builtin type which is defined outside of the argument-defining class body the type can't be found which results in NameErrors. This is avoidable either by only using custom types which have been defined in the argument-defining class body (which is restrictive), or alternatively by not using the annotations import which should not be necessary from python 3.13 forward thanks to PEP 649.