Python by Structure：上下文管理器与 with 语句

Source: Dev.to

手动文件处理的问题

Timothy 在调试一个文件处理脚本时，Margaret 在日志中注意到一个 “打开的文件过多” 错误。

def process_config():
    f = open('config.txt', 'r', encoding='utf-8')
    data = f.read()
    # Process data...
    if 'error' in data:
        return None  # Oops – file never closed!
    f.close()
    return data

如果走到早期的 return，文件将保持打开状态。依赖垃圾回收不可靠，且可能耗尽系统资源。

传统解决方案：try/finally

def process_config():
    f = open('config.txt', 'r', encoding='utf-8')
    try:
        data = f.read()
        if 'error' in data:
            return None
        return data
    finally:
        f.close()

finally 块保证无论函数如何退出，f.close() 都会执行，但额外的样板代码令人烦恼。

with 语句

Python 的 with 语句自动完成清理工作：

def process_config():
    with open('config.txt', 'r', encoding='utf-8') as f:
        data = f.read()
        if 'error' in data:
            return None
        return data
# File is automatically closed here

with 块有明确的入口点（打开文件）和退出点（缩进块结束）。Python 保证当块被离开时文件会被关闭，无论是正常执行、提前 return，还是抛出异常。

with 的工作原理：上下文管理器

在 with 语句中使用的对象必须实现两个特殊方法：

• __enter__(self): 在块开始时调用；其返回值绑定到 as 后面的变量。
• __exit__(self, exc_type, exc_val, exc_tb): 在块结束时调用；如果抛出了异常，会收到异常信息。

简单的上下文管理器示例

class FileLogger:
    def __init__(self, filename):
        self.filename = filename
        self.file = None

    def __enter__(self):
        print(f"Opening {self.filename}")
        self.file = open(self.filename, 'w', encoding='utf-8')
        return self.file

    def __exit__(self, exc_type, exc_val, exc_tb):
        print(f"Closing {self.filename}")
        if self.file:
            self.file.close()
        return False  # Do not suppress exceptions

# Usage
with FileLogger('output.log') as log:
    log.write('Starting process\n')
    log.write('Processing data\n')
# File automatically closed here

输出

Opening output.log
Closing output.log

__exit__ 中的异常处理

如果在 with 块内部出现异常，Python 会把异常细节传递给 __exit__。该方法可以通过返回 True 来决定是否抑制异常。

class ErrorLogger:
    def __enter__(self):
        print("Entering context")
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        if exc_type is not None:
            print(f"Exception occurred: {exc_type.__name__}: {exc_val}")
            return True   # Suppress the exception
        print("Exiting normally")
        return False

with ErrorLogger():
    print("Working...")
    raise ValueError("Something went wrong!")
print("Continuing after exception")

输出

Entering context
Working...
Exception occurred: ValueError: Something went wrong!
Continuing after exception

使用多个 with 语句

可以嵌套 with 语句，或在单行中组合使用（Python 3.1+）。

# Nested
with open('input.txt', 'r', encoding='utf-8') as infile:
    with open('output.txt', 'w', encoding='utf-8') as outfile:
        outfile.write(infile.read())

# Combined
with open('input.txt', 'r', encoding='utf-8') as infile, \
     open('output.txt', 'w', encoding='utf-8') as outfile:
    outfile.write(infile.read())

两个文件都保证被关闭，关闭顺序与获取顺序相反（后打开的先关闭）。

上下文管理器的常见用例

• 文件操作 – 保证关闭
• 数据库连接 – 保证提交/回滚
• 锁和信号量 – 保证释放
• 网络连接 – 保证断开
• 临时状态更改 – 保证恢复
• 任何需要清理的资源

示例：线程锁

import threading

lock = threading.Lock()

# Without context manager – risky
lock.acquire()
try:
    # Critical section
    pass
finally:
    lock.release()

# With context manager – safe
with lock:
    # Critical section
    pass

即使在关键区块内部抛出异常，锁也会自动释放。