Python 学习手册

# 实现上下文管理器需要 __enter__ 和 __exit__
class ManagedFile:
    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()

with ManagedFile("hello.txt", "w") as f:
    f.write("Hello, World!")

# 使用 contextlib 简化
from contextlib import contextmanager
@contextmanager
def open_file(filename, mode):
    f = open(filename, mode)
    try:
        yield f
    finally:
        f.close()