上下文管理器（with）

# ① 协议类
class Timer:
    def __enter__(self):
        self.t0 = time.perf_counter()
        return self                      # as 变量拿到的是 __enter__ 的返回值
    def __exit__(self, exc_type, exc, tb):
        print(f"耗时 {time.perf_counter()-self.t0:.3f}s")
        return False                     # False/None：异常继续传播；True：吞掉

with Timer():
    heavy()

# ② contextlib：临时上下文的最小成本写法
from contextlib import contextmanager

@contextmanager
def timer():
    t0 = time.perf_counter()
    try:
        yield                            # yield 之前 = __enter__，之后 = __exit__
    finally:
        print(f"耗时 {time.perf_counter()-t0:.3f}s")

# ③ 标准库常用工具
import contextlib, tempfile
with contextlib.suppress(FileNotFoundError):   # 静默特定异常
    os.remove("tmp.txt")
with open("a") as fa, open("b") as fb: ...     # 多资源，3.1+ 可写括号组
with contextlib.ExitStack() as stack:          # 动态数量的资源
    files = [stack.enter_context(open(p)) for p in paths]

with obj as x:
    body
等价于：
  mgr = obj
  x = mgr.__enter__()
  try:
      body
  except BaseException as e:
      if not mgr.__exit__(type(e), e, e.__traceback__): raise
  else:
      mgr.__exit__(None, None, None)

进入 with ──► __enter__（可返回值给 as）
退出 with ──► __exit__ 必被调用（正常/异常都算"退出"）
生成器版：yield 前后的代码天然对应 enter/exit

# 数据库事务模板
@contextmanager
def transaction(conn):
    try:
        yield conn
        conn.commit()
    except Exception:
        conn.rollback()
        raise                            # 回滚后继续抛出，调用方知情