validate with per-field error messages

"""Validate payload fields and collect one clear message per bad field."""
import re
from typing import Any, Callable

Check = Callable[[Any], bool]
Rules = dict[str, tuple[Check, str]]  # field -> (predicate, failure hint)

EMAIL_RE = re.compile(r"^[^@\s]+@[^@\s]+\.[a-z]{2,}$", re.IGNORECASE)


def validate(payload: dict[str, Any], rules: Rules, required: set[str]) -> dict[str, str]:
    """Return {field: message} for failing fields; empty dict means valid."""
    errors: dict[str, str] = {}
    for field, (check, hint) in rules.items():
        value = payload.get(field)
        if value is None or (isinstance(value, str) and not value.strip()):
            if field in required:
                errors[field] = f"{field} is required"
            continue
        if not check(value):
            errors[field] = f"{field} must {hint}"
    return errors


def is_email(value: Any) -> bool:
    return isinstance(value, str) and EMAIL_RE.match(value) is not None


def between(low: float, high: float) -> Check:
    """Predicate factory for numeric ranges (bools are rejected)."""
    def check(value: Any) -> bool:
        if isinstance(value, bool) or not isinstance(value, (int, float)):
            return False
        return low <= value <= high
    return check