Validate Environment Config At Startup

import os
from dataclasses import dataclass, field
from typing import Any, Callable, Dict, List, Mapping, Optional


class ConfigError(ValueError):
    """Raised when environment configuration is invalid."""


@dataclass(frozen=True)
class EnvVar:
    name: str
    parser: Callable[[str], Any]
    required: bool = True
    default: Optional[Any] = None
    minimum: Optional[float] = None
    maximum: Optional[float] = None


@dataclass(frozen=True)
class Config:
    values: Dict[str, Any] = field(default_factory=dict)

    def __getitem__(self, key: str) -> Any:
        return self.values[key]


def load_config(schema: List[EnvVar], env: Optional[Mapping[str, str]] = None) -> Config:
    """Load and validate configuration from env, reporting all problems at once."""
    source = os.environ if env is None else env
    problems: List[str] = []
    values: Dict[str, Any] = {}

    for var in schema:
        raw = source.get(var.name)
        if raw is None or raw == "":
            if var.required:
                problems.append(f"{var.name}: required but missing")
            else:
                values[var.name] = var.default
            continue
        try:
            parsed = var.parser(raw)
        except (ValueError, TypeError) as exc:
            problems.append(f"{var.name}: cannot parse {raw!r} ({exc})")
            continue
        if isinstance(parsed, (int, float)):
            if var.minimum is not None and parsed < var.minimum:
                problems.append(f"{var.name}: {parsed} below minimum {var.minimum}")
                continue
            if var.maximum is not None and parsed > var.maximum:
                problems.append(f"{var.name}: {parsed} above maximum {var.maximum}")
                continue
        values[var.name] = parsed

    if problems:
        raise ConfigError("invalid configuration:\n  " + "\n  ".join(problems))
    return Config(values=values)