上下文管理

上下文构造

• 调用方式

with xxxx as xxx:
    do.....

对象形式

• __enter__
• __exit__

class Open:
    def __init__(self):
        pass
        
    def __enter__(self, file):
        return f = open(file, 'w')
        
    def __exit__(self):
        f.close()
        return
  
with custom_open('file') as f:
    contents = f.read()

装饰器形式

• contextlib
• contextlib.contextmanager

from contextlib import contextmanager
 
@contextmanager
def custom_open(filename):
    f = open(filename)
    try:
        yield f
    finally:
        f.close()
 
with custom_open('file') as f:
    contents = f.read()

如何选取：

• 需要处理的逻辑负责，代码量大的使用类包装
• 需要处理的逻辑简单，代码量小的使用装饰器