1.https://www.cnblogs.com/flashBoxer/p/9664813.html

python中的__enter__ __exit__

上下文管理器是支持上下文管理器协议的对象，它为了with而生。

with语句开始运行时，会在上下文管理器对象上调用__enter__方法

               运行结束后，                                                  __exit__方法

 
class Timer:
    def __init__(self, msg):
        self.msg = msg
        self.start_time = None
 
    def __enter__(self):
        self.start_time = time.time()#enter的时间
 
    def __exit__(self, exc_type, exc_value, exc_tb):
        print(self.msg % (time.time() - self.start_time))

     with Timer("Elapsed time in update: %f"):
            # Main training code

对文件的操作 

先前：

try:
    f = open('filename')
except:
    print("Unexpected error:", sys.exc_info()[0])
else:
    print(f.readlines())
    f.close()

现在：

用with语句，代码更简洁，减少编码量,在执行完后自动关闭文件(即使出现异常也会)

with open('example.info', 'r') as f:
    print(f.readlines())

例子：

class TmpTest:
    def __init__(self,filename):
        self.filename=filename
    def __enter__(self):
        self.f = open(self.filename, 'r')
        return self.f#不能省略 否则报错
    def __exit__(self, exc_type, exc_val, exc_tb):
        self.f.close()
 
test=TmpTest('file')
 
with test as t:# t = __enter__的返回值
    print ('test result: {}'.format(t))