Injectable Clock For Tests

from dataclasses import dataclass
from datetime import datetime, timedelta, timezone
from typing import Protocol


class Clock(Protocol):
    def now(self) -> datetime: ...


class SystemClock:
    def now(self) -> datetime:
        return datetime.now(timezone.utc)


class FakeClock:
    def __init__(self, start: datetime) -> None:
        if start.tzinfo is None:
            raise ValueError("start must be timezone aware")
        self._current = start

    def now(self) -> datetime:
        return self._current

    def advance(self, seconds: float) -> None:
        self._current += timedelta(seconds=seconds)


@dataclass
class CachedValue:
    clock: Clock
    ttl: timedelta
    value: str | None = None
    expires_at: datetime | None = None

    def get(self) -> str | None:
        now = self.clock.now()
        if self.value is None or self.expires_at is None or now >= self.expires_at:
            return None
        return self.value

    def put(self, value: str) -> None:
        self.value = value
        self.expires_at = self.clock.now() + self.ttl


def main() -> None:
    clock = FakeClock(datetime(2026, 10, 21, 12, 0, tzinfo=timezone.utc))
    cache = CachedValue(clock=clock, ttl=timedelta(seconds=30))
    cache.put("fresh")
    print("immediately:", cache.get())
    clock.advance(29)
    print("after 29s:", cache.get())
    clock.advance(2)
    print("after 31s:", cache.get())
    print("system clock tz:", SystemClock().now().tzinfo)


if __name__ == "__main__":
    main()