关于退出返回值:

If the suite was exited due to an exception, and the return value from the __exit__() method was false, the exception is reraised. If the return value was true, the exception is suppressed, and execution continues with the statement following the with statement.

If the suite was exited for any reason other than an exception, the return value from __exit__() is ignored, and execution proceeds at the normal location for the kind of exit that was taken.

中文：
如果套件由于异常而退出，并且__exit __（）方法的返回值为false，则会重新对异常进行重新评估。 如果返回值为true，则异常被抑制，并继续执行with语句后面的语句。

如果套件由于除了异常之外的任何原因而退出，则__exit __（）的返回值将被忽略，并且执行将在正常位置继续进行。

意思就是：

如果是异常退出，那么会返回false,(根据文档中的exit的描述“that __exit__() methods should not reraise the passed-in exception; this is the caller’s responsibility.”，大概意思就是exit()不会处理异常，会重新抛出异常抛出给外面，由调用者处理，因为这是调用者的责任)

如果返回 True，则忽略异常，不再对异常进行处理【（在exit内部处理完异常后，可以让”__exit__()”方法返回True，此时该异常就会不会再被抛出,with会认为它的执行体没有发生异常)】

import time
class myContextDemo(object):
    def __init__(self,gen):
        self.gen = gen
    def __enter__(self):
        print("enter in ")
        return self.gen
    def __exit__(self, exc_type, exc_val, exc_tb):
#exc_type是exception_type  exc_val是exception_value  exc_tb是exception_trackback
        print("exit in ")
        if exc_type is None:#如果是None 则继续执行
            print("None:",exc_type, exc_val, exc_tb)

        else: #异常不为空时执行，这一步，如果with语句体中发生异常，那么也会执行
            print("exception:", exc_type, exc_val, exc_tb)
            print("all done")
        


if __name__=="__main__":
    gen=(i for i in range(5,10))
    G=myContextDemo(gen)
    with G as f :
        print("hello")
        for i in f:
            print(i,end="\t")
        #测试1：执行体中发生异常
        raise Exception("母鸡啊")
    print("main continue")

import time
class myContextDemo(object):
    def __init__(self,gen):
        self.gen = gen
    def __enter__(self):
        print("enter in ")
        return self.gen
    def __exit__(self, exc_type, exc_val, exc_tb):
#exc_type是exception_type  exc_val是exception_value  exc_tb是exception_trackback
        print("exit in ")
        if exc_type is None:#如果是None 则继续执行
            print("None:",exc_type, exc_val, exc_tb)

        else: #异常不为空时执行，这一步，如果with语句体中发生异常，那么也会执行
            print("exception:", exc_type, exc_val, exc_tb)
            print("all done")
            return True #这里如果返回true可以看到发生异常后，main continue可以执行
            #即，如果exc_type是true，那么会继续执行，实际上，也可以在这里处理一下异常再返回true


if __name__=="__main__":
    gen=(i for i in range(5,10))
    G=myContextDemo(gen)
    with G as f :
        print("hello")
        for i in f:
            print(i,end="\t")
        raise Exception("母鸡啊")
        # print("continue")#这里不会执行
    print("main continue")