Environnement variable fallback annotation

[jRimbault]

Hi, what do you think of adding a tyro.conf.Env to the available annotations?

It would be used something like this:

@dataclasses.dataclass
class ServerConfig:
    # Env var name derived automatically from field name. Required.
    secret: Annotated[str, tyro.conf.Env()]

    # Falls back to HOST env var if --host is not passed.
    host: Annotated[str, tyro.conf.Env()] = "localhost"

    # Explicit env var name overrides derivation.
    port: Annotated[int, tyro.conf.Env("APP_PORT")] = 8080

Priority would be: hardcoded < environment < flags.

[brentyi]

Hi @jRimbault!

I see why this might be useful and put a draft implementation in #456.

Not 100% sure I want to merge though, there are "how many people use this?" / feature creep / maintenance burden questions that I'd want to stew on for a bit.

[jRimbault]

Completely understandable, each new features brings in complexity.

[brentyi]

Here's a pattern that works with the current tyro API, do you think it'd be enough for your needs?

import dataclasses
import os
from typing import Annotated, Any

import tyro

def env(var_name: str) -> Any:
    """Returns tyro metadata for loading default from environment variable."""
    return tyro.conf.arg(default=os.environ.get(var_name, tyro.MISSING))

@dataclasses.dataclass
class ServerConfig:
    secret: Annotated[str, env("SECRET")]

tyro.cli(ServerConfig)

The main thing it misses is automatic variable naming. But it's nice to be explicit anyways 🙂

[jRimbault]

That's very close to what I use in practice ^^ (basically only symbols' names and docstring differ)