场景

资源是有限的，使用过后需要释放，否则会造成资源泄露

File

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for x in range(10_000_000):
    f = open('test.txt', 'w')  # OSError: [Errno 24] Too many open files
    f.write('hello')

context manager 帮助自动分配并释放资源 - with 语句

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for x in range(10_000_000):
    with open('test.txt', 'w') as f:  # This will open and close the file 10_000_000 times
        f.write('hello')

try - finally

f = open('test.txt', 'w')
try:
    f.write('hello')
finally:
    f.close()  # close the file even if an exception occurs

try - finally

import threading

some_lock = threading.Lock()
some_lock.acquire()
try:
    ...
finally:
    some_lock.release()

with

import threading

some_lock = threading.Lock()
with some_lock:  # releases the lock when the block is exited
    ...

基于类的上下文管理器

发生异常

基于生成器的上下文管理器

使用 contextlib.contextmanager 来定义基于生成器的上下文管理器，用来支持 with 语句
file_manager() 函数是一个生成器
- 执行 with 语句时，便会打开文件，并返回文件对象
- 当 with 语句执行完毕后，执行 finally block 中关闭文件操作