Custom Constructors

For additional flexibility, tyro.conf.arg() accepts a constructor argument, which makes it easier to load complex objects.

import json as json_

from typing_extensions import Annotated

import tyro


def dict_json_constructor(json: str) -> dict:
    """Construct a dictionary from a JSON string. Raises a ValueError if the result is
    not a dictionary."""
    out = json_.loads(json)
    if not isinstance(out, dict):
        raise ValueError(f"{json} is not a dictionary!")
    return out


# A dictionary type, but `tyro` will expect a JSON string from the CLI.
JsonDict = Annotated[dict, tyro.conf.arg(constructor=dict_json_constructor)]


def main(
    dict1: JsonDict,
    dict2: JsonDict = {"default": None},
) -> None:
    print(f"{dict1=}")
    print(f"{dict2=}")


if __name__ == "__main__":
    tyro.cli(main)

---

python 04_additional/11_custom_constructors.py --help

usage: 11_custom_constructors.py [-h] --dict1.json STR [--dict2.json STR]

╭─ options ──────────────────────────────────────────────────────────────────╮
│ -h, --help              show this help message and exit                    │
╰────────────────────────────────────────────────────────────────────────────╯
╭─ dict1 options ────────────────────────────────────────────────────────────╮
│ Construct a dictionary from a JSON string. Raises a ValueError if the      │
│ result is                                                                  │
│ not a dictionary.                                                          │
│ ────────────────────────────────────────────────────────────────────────── │
│ --dict1.json STR        (required)                                         │
╰────────────────────────────────────────────────────────────────────────────╯
╭─ dict2 options ────────────────────────────────────────────────────────────╮
│ Construct a dictionary from a JSON string. Raises a ValueError if the      │
│ result is                                                                  │
│ not a dictionary.                                                          │
│                                                                            │
│ Default: {'default': None}                                                 │
│ ────────────────────────────────────────────────────────────────────────── │
│ --dict2.json STR        (optional)                                         │
╰────────────────────────────────────────────────────────────────────────────╯

---

python 04_additional/11_custom_constructors.py --dict1.json '{"hello": "world"}'

dict1={'hello': 'world'}
dict2={'default': None}

---

python 04_additional/11_custom_constructors.py --dict1.json '{"hello": "world"}`' --dict2.json '{"hello": "world"}'

╭─ Value error ────────────────────────────────────────────────────────────────╮
│ Error parsing dict1: Extra data: line 1 column 19 (char 18)                  │
╰──────────────────────────────────────────────────────────────────────────────╯