Basic start/stop timer context manager

timer.py

# TODO: Thread & async safety

import sys
import time
from collections.abc import Callable

if sys.version_info >= (3, 11):
    from typing import Self
else:
    from typing_extensions import Self


class Timer:
    time: Callable[[], float]
    t0: float | None
    t1: float | None

    def __init__(self, time: Callable[[], float] = time.perf_counter) -> None:
        self.time = time
        self.t0 = None
        self.t1 = None

    def reset(self) -> None:
        self.t0 = None
        self.t1 = None

    def start(self) -> None:
        if self.t0 is not None:
            raise RuntimeError("Timer is already running!")
        self.t0 = self.time()

    def mark(self) -> None:
        if self.t0 is None:
            raise RuntimeError("Timer was not started!")
        self.t1 = self.time()

    def elapsed(self) -> None:
        if self.t0 is None:
            raise RuntimeError("Timer was not started!")
        if self.t1 is not None:
            raise RuntimeError("Timer was not marked!")
        return self.t1 - self.t0

    def __enter__(self) -> Self:
        self.start()

    def __exit__(self, *_: object) -> None:
        self.mark()