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“刘心向学（38）：enter与 exit：深入理解上下文管理器”

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Dear, this is LearningYard Academy.

Today, the editor brings you an article.

“Liu Xinxiangxue (38): __enter__ vs __exit__: Understanding Context Managers in Python”

Welcome to your visit.

一、思维导图（Mind Map）

二、引言（Introduction）

在 Python 编程中，资源管理是一个非常重要的话题。比如打开文件、连接数据库、获取锁、网络连接等操作，都需要在使用完资源后 正确释放，否则可能会导致资源泄露、程序崩溃或性能下降。

In Python programming, resource management is a very important topic. For example, operations such as opening files, connecting to a database, acquiring locks, and network connections all require the resources to be properly released after use. Otherwise, it may lead to resource leaks, program crashes, or performance degradation.

Python 提供了一种优雅的机制来处理这类问题 —— 上下文管理器（Context Manager），其核心就是 __enter__ 和 __exit__ 这两个魔法方法。

Python provides an elegant mechanism to handle such issues — the context manager, whose core consists of the two magic methods __enter__ and __exit__.

本文将带你深入了解 __enter__ 和 __exit__ 的作用、使用方式、调用流程，并通过多个实际示例展示它们在文件操作、锁管理、异常处理等场景中的应用。同时，我们还将介绍如何使用 contextlib 模块简化上下文管理器的编写。

This article will help you deeply understand the roles, usage, and invocation flow of __enter__ and __exit__, and demonstrate their applications in scenarios such as file operations, lock management, and exception handling through multiple practical examples. In addition, we will also introduce how to use the contextlib module to simplify the writing of context managers.

三、什么是上下文管理器？（What is a Context Manager?）

上下文管理器是一种设计模式，用于 在代码执行前后自动执行某些操作，例如：

A context manager is a design pattern used to automatically perform certain operations before and after code execution, such as:

在进入时打开资源

在退出时关闭资源

Opening resources when entering

Closing resources when exiting

它通过 __enter__ 和 __exit__ 方法实现，常与 with 语句一起使用。

It is implemented through the __enter__ and __exit__ methods, and is commonly used with the with statement.

基本语法：

Basic Syntax:

Python深色版本withopen('file.txt', 'r') as f:
    content = f.read()

在这个例子中，open() 返回的对象就是一个上下文管理器。它在进入时打开文件，在退出时自动关闭文件。

In this example, the object returned by open() is a context manager. It opens the file when entering and automatically closes it when exiting.

四、__enter__与 __exit__方法详解（Detailed Explanation of __enter__and __exit__Methods）

1. __enter__方法

1. The __enter__Method

在进入 with 块之前被调用。

It is called before entering the with block.

通常用于初始化资源。

Usually used to initialize resources.

返回值会绑定到 as 后的变量。

Its return value is bound to the variable after as.

__exit__ 方法

2. The __exit__ Method

在 with 块结束时被调用（无论是否发生异常）。

It is called at the end of the with block (regardless of whether an exception occurred).

通常用于释放资源。

Usually used to release resources.

接受三个参数：exc_type, exc_val, exc_tb，分别表示异常类型、值和回溯信息。

It accepts three arguments: exc_type, exc_val, and exc_tb, which represent the exception type, value, and traceback information respectively.

自定义上下文管理器示例：

Custom Context Manager Example:

Python深色版本classMyContextManager:
def__enter__(self):
print("Entering the context")
return self

def__exit__(self, exc_type, exc_val, exc_tb):
print("Exiting the context")
if exc_type:
print(f"An exception occurred: {exc_val}")
returnTrue# 返回 True 表示已处理异常，不继续抛出

with MyContextManager() as cm:
print("Inside the context")
raise ValueError("Something went wrong")

输出：

Output:

深色版本Entering the context
Inside the context
Exiting the context
An exception occurred: Something went wrong

五、典型应用场景（Typical Use Cases）

1. 文件操作（标准库支持）File Operations (Standard Library Support)

Python深色版本withopen('data.txt', 'r') as f:
    data = f.read()
# 文件自动关闭

2. 网络连接管理

Network Connection Management

Python深色版本import socket

classSocketContextManager:
def__init__(self, host, port):
        self.sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        self.host = host
        self.port = port

def__enter__(self):
        self.sock.connect((self.host, self.port))
return self.sock

def__exit__(self, *args):
        self.sock.close()

with SocketContextManager('example.com', 80) as sock:
    sock.sendall(b'GET / HTTP/1.1\r\nHost: example.com\r\n\r\n')
    response = sock.recv(4096)
print(response)

3. 上下文中的锁管理（线程安全）

Lock Management in Context (Thread-safe)

Python深色版本import threading

classLockContextManager:
def__init__(self):
        self.lock = threading.Lock()

def__enter__(self):
        self.lock.acquire()
return self

def__exit__(self, *args):
        self.lock.release()

with LockContextManager():
# 安全地访问共享资源
print("Accessing shared resource")

4. 数据库连接池管理

Database Connection Pool Management

Python深色版本classDBConnection:
def__init__(self):
        self.conn = None

defconnect(self):
        self.conn = "Connected to DB"
print("Connected to database")

defclose(self):
        self.conn = None
print("Connection closed")

def__enter__(self):
        self.connect()
return self

def__exit__(self, *args):
        self.close()

with DBConnection() as db:
print(db.conn)

六、使用 contextlib简化上下文管理器（Simplifying Context Managers with contextlib）

Python 标准库 contextlib 提供了装饰器 @contextmanager，可以更简单地创建上下文管理器，而无需手动实现 __enter__ 和 __exit__。

The Python standard library contextlib provides the @contextmanager decorator, which simplifies the creation of context managers without manually implementing __enter__ and __exit__.

示例：

Example:

Python深色版本from contextlib import contextmanager

@contextmanager
defsimple_context():
print("Entering")
try:
yield"Resource"
finally:
print("Exiting")

with simple_context() as res:
print(f"Using {res}")

输出：

Output:

深色版本Entering
Using Resource
Exiting

七、注意事项（Notes）

__exit__ 必须处理异常：即使发生异常，也要确保资源释放。

The __exit__ method must handle exceptions: even if an exception occurs, resources must be ensured to be released.

不要在 __exit__ 中抛出异常：除非你有意要中断流程。

Do not raise exceptions in __exit__: unless you intend to interrupt the flow.

使用 contextlib 更简洁：适用于简单的上下文逻辑。

Use contextlib for more concise code: suitable for simple context logic.

避免在 __enter__ 中做耗时操作：影响性能。

Avoid time-consuming operations in __enter__: affects performance.

八、结语（Conclusion）

__enter__ 和 __exit__ 是 Python 中非常强大的魔法方法，它们构成了上下文管理器的核心机制。通过 with 语句，我们可以实现资源的自动管理、异常的安全处理以及代码的清晰结构。

__enter__ and __exit__ are very powerful magic methods in Python that form the core mechanism of context managers. With the with statement, we can achieve automatic resource management, safe exception handling, and clear code structure.

掌握上下文管理器的使用，不仅能提升代码的健壮性和可维护性，还能让你更深入地理解 Python 的对象生命周期管理和资源控制机制。

Mastering the use of context managers not only improves the robustness and maintainability of your code but also allows you to better understand Python's object lifecycle management and resource control mechanisms.

无论你是开发 Web 应用、处理文件系统、构建并发程序，还是封装底层资源调用，__enter__ 和 __exit__ 都是你必须掌握的重要技能。

Whether you're developing web applications, working with file systems, building concurrent programs, or encapsulating low-level resource calls, __enter__ and __exit__ are essential skills you must master.

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参考资料：通义千问

参考文献：Beazley, D., & Jones, B. K. (2019). Python Cookbook (3rd ed.). O'Reilly Media.

Hettinger, R. (2019). Transforming Code into Beautiful, Idiomatic Python. PyCon US.

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