Python实现上下文协议，自定义with对象

import time
<p>class Timer:
def <strong>init</strong>(self, name="block"):
self.name = name
self.start_time = None
self.end_time = None</p><pre class="brush:php;toolbar:false"><code>def __enter__(self):
    self.start_time = time.time()
    return self  # 可选：返回自身或任意值供 as 使用

def __exit__(self, exc_type, exc_value, traceback):
    self.end_time = time.time()
    elapsed = self.end_time - self.start_time
    print(f"[{self.name}] 执行耗时: {elapsed:.4f} 秒")
    # 不返回 True，不抑制异常</code>

with Timer("数据处理"):
    time.sleep(1.2)

class SafeFileOpener:
    def __init__(self, filename, mode="r"):
        self.filename = filename
        self.mode = mode
        self.file = None
<pre class="brush:php;toolbar:false"><code>def __enter__(self):
    self.file = open(self.filename, self.mode)
    return self.file

def __exit__(self, exc_type, exc_value, traceback):
    if self.file:
        self.file.close()
    if exc_type is FileNotFoundError:
        print(f"警告：文件 {self.filename} 不存在，已跳过")
        return True  # 抑制 FileNotFoundError
    return False  # 其他异常照常抛出</code>

with SafeFileOpener("missing.txt") as f:
    content = f.read()  # 若文件不存在，只打印警告，不报错

from contextlib import contextmanager
<p>@contextmanager
def timer(name="block"):
start = time.time()
try:
yield  # 进入 with 块的位置
finally:
end = time.time()
print(f"[{name}] 耗时: {end - start:.4f} 秒")</p><h1>使用方式相同</h1><p>with timer("循环计算"):
sum(i * i for i in range(10**6))
</p>