上下文管理器 - with

• 自动处理错误?

# 不用写 try
with open('name.txt') as f:
    for i in f:
        print(i)

• 类似装饰器

class TestWith:
    def __enter__(self):
        print('enter')
    def __exit__(self, exc_type, exc_val, exc_tb):
        print('exit')
with TestWith():
    print('Test is running')
# 输出
enter
Test is running
exit

• 捕获类的异常

class TestWith:
    def __enter__(self):
        print('enter')
    def __exit__(self, exc_type, exc_val, exc_tb):
        if exc_tb is None:
            print('normal exit')
        else:
            print('err: %s' % exc_tb)
# 可以想像成 with 包裹了 TestWith 对象,
# 如果在运行中产生异常, 那么异常会传递给对象.
with TestWith():
    print('Test is running')
    # 手动抛出异常
    raise NameError('testNameError')
# 输出
enter
Test is running
err: <traceback object at 0x7fe36e8614c0>
Traceback (most recent call last):
  File "/home/jdxj/workspace/study-python/with.py", line 15, in <module>
    raise NameError('testNameError')
NameError: testNameError