Python上下文管理器的用法(Context Manager)

with context_manager as resource:
    # 使用资源的代码块

file = open('example.txt', 'r')
try:
    data = file.read()
finally:
    file.close()  # 必须确保文件被关闭

with open('example.txt', 'r') as file:
    data = file.read()  # 文件会自动关闭

class FileManager:
    def __init__(self, filename, mode):
        self.filename = filename
        self.mode = mode
        
    def __enter__(self):
        self.file = open(self.filename, self.mode)
        return self.file
        
    def __exit__(self, exc_type, exc_val, exc_tb):
        self.file.close()
        if exc_type:  # 如果有异常发生
            print(f"异常发生: {exc_val}")
        return True  # 返回True表示已处理异常

# 使用示例
with FileManager('test.txt', 'w') as f:
    f.write('Hello Context Manager!')

from contextlib import contextmanager

@contextmanager
def file_manager(filename, mode):
    try:
        file = open(filename, mode)
        yield file  # yield之前的代码相当于__enter__
    finally:
        file.close()  # yield之后的代码相当于__exit__

# 使用示例
with file_manager('test.txt', 'w') as f:
    f.write('Hello from contextlib!')

with open('data.csv', 'r') as csvfile:
    reader = csv.reader(csvfile)
    for row in reader:
        process(row)

with psycopg2.connect(DATABASE_URL) as conn:
    with conn.cursor() as cursor:
        cursor.execute("SELECT * FROM users")
        results = cursor.fetchall()

lock = threading.Lock()

with lock:
    # 线程安全的代码区域
    shared_resource += 1

with mock.patch('module.function', return_value=42):
    # 在这个块中，module.function()会返回42
    result = module.function()

with open('input.txt') as fin, open('output.txt', 'w') as fout:
    fout.write(fin.read())

class AsyncConnection:
    async def __aenter__(self):
        self.conn = await create_connection()
        return self.conn
        
    async def __aexit__(self, exc_type, exc, tb):
        await self.conn.close()

async with AsyncConnection() as conn:
    await conn.execute(...)

from contextlib import ExitStack

with ExitStack() as stack:
    files = [stack.enter_context(open(fname)) for fname in filenames]
    # 所有文件都会在退出时自动关闭

with manager() as (a, b, c):
    # 使用a, b, c

class Transaction:
    def __init__(self, db):
        self.db = db
        
    def __enter__(self):
        self.conn = self.db.get_connection()
        self.conn.begin()
        return self.conn
        
    def __exit__(self, exc_type, exc_val, exc_tb):
        if exc_type is None:
            self.conn.commit()
        else:
            self.conn.rollback()
        self.conn.close()

# 使用示例
with Transaction(database) as conn:
    conn.execute("UPDATE accounts SET balance = balance - 100 WHERE id = 1")
    conn.execute("UPDATE accounts SET balance = balance + 100 WHERE id = 2")