28. with 语句与上下文管理器

# 打开文件会获取一种资源（文件句柄）
file = open("data.txt", "r")
content = file.read()
# 使用该文件...
file.close()  # 释放资源

# 文件对象是上下文管理器
# 它们有 __enter__ 和 __exit__ 方法
file = open("example.txt", "r")
print(hasattr(file, "__enter__"))  # Output: True
print(hasattr(file, "__exit__"))   # Output: True
file.close()

# 读取配置文件
config_file = open("config.txt", "r")
settings = config_file.read()
# 糟糕！忘记关闭文件
# 文件句柄仍然保持打开状态

# 尝试处理文件
data_file = open("data.txt", "r")
content = data_file.read()
result = process_data(content)  # 如果这里抛出错误怎么办？
data_file.close()  # 如果 process_data() 失败，这一行永远不会执行！

def read_first_valid_line(filename):
    file = open(filename, "r")
    
    for line in file:
        line = line.strip()
        if line and not line.startswith("#"):
            # 找到一行有效内容——但文件仍然是打开的！
            return line
    
    file.close()  # 只有在没有找到有效行时才会执行到这里
    return None

# 尝试正确处理错误
file = None
try:
    file = open("data.txt", "r")
    content = file.read()
    result = process_data(content)
except FileNotFoundError:
    print("File not found")
except ValueError:
    print("Invalid data format")
finally:
    if file is not None:
        file.close()

# 警告：资源泄漏——仅用于演示
# 问题：文件从未被关闭
def process_many_files(filenames):
    results = []
    for filename in filenames:
        file = open(filename, "r")  # 打开一个文件
        data = file.read()
        results.append(analyze(data))
        # 错误：从不关闭文件
    return results
 
# 处理 1000 个文件后，你就有 1000 个打开的文件句柄！
# 最终，操作系统会拒绝打开更多文件

OSError: [Errno 24] Too many open files: 'file_1001.txt'

with expression as variable:
    # 使用资源的代码块
    # 缩进在 with 语句之下
# 资源在这里自动释放

# 手动方式（有风险）
file = open("data.txt", "r")
content = file.read()
file.close()
 
# 使用 with 语句（安全）
with open("data.txt", "r") as file:
    content = file.read()
# 文件在这里自动关闭，即使发生错误也是如此

# 从一个文件读取并写入另一个文件
with open("input.txt", "r") as input_file, open("output.txt", "w") as output_file:
    for line in input_file:
        processed = line.upper()
        output_file.write(processed)
# 两个文件都在这里自动关闭

# 嵌套 with 语句（等价但更冗长）
with open("input.txt", "r") as input_file:
    with open("output.txt", "w") as output_file:
        for line in input_file:
            processed = line.upper()
            output_file.write(processed)

import gzip
 
# 写入压缩数据
with gzip.open("data.txt.gz", "wt") as compressed_file:
    compressed_file.write("This text will be compressed\n")
    compressed_file.write("Saving space on disk\n")
# 文件自动关闭并完成压缩
 
# 读取压缩数据
with gzip.open("data.txt.gz", "rt") as compressed_file:
    content = compressed_file.read()
    print(content)

This text will be compressed
Saving space on disk

import os
 
# 当前目录
print(f"Starting in: {os.getcwd()}")
 
# 手动切换目录（有风险）
original_dir = os.getcwd()
os.chdir("/tmp")
print(f"Now in: {os.getcwd()}")
process_files()  # 如果这里发生错误，我们可能无法回到 original_dir
os.chdir(original_dir)

import os
from contextlib import chdir
 
print(f"Starting in: {os.getcwd()}")
 
# 使用上下文管理器（安全）
with chdir("/tmp"):
    print(f"Temporarily in: {os.getcwd()}")
    process_files()  # 即使这里抛出错误，我们也会回到原始目录
    
print(f"Back in: {os.getcwd()}")
# 自动回到原始目录

# 概念示例（我们会在高级主题中学习 threading）
import threading
 
lock = threading.Lock()
 
# 手动管理锁（有风险）
lock.acquire()
# 临界区——如果发生错误怎么办？
lock.release()  # 可能不会执行
 
# 使用 with 语句（安全）
with lock:
    # 临界区
    # 锁会自动释放，即使发生错误也是如此
    pass

with open("data.txt", "r") as file:
    content = file.read()
    print(content)

# 发生错误时会怎样
try:
    with open("data.txt", "r") as file:
        content = file.read()
        result = int(content)  # 可能抛出 ValueError
        print(result)
except ValueError as e:
    print(f"Invalid data: {e}")
# 在 except 代码块运行之前，文件已经关闭

with resource_manager as resource:
    # 使用资源
    pass
# Python 保证清理已经发生