永远不要让用户看见大黄页

def foo(x):
    x = x + 1
    print (x) 

try:
    foo(rf)
    foo(2)
    
except Exception as a:  #  except:
    print(a)

else:
    print("It's OK")

finally:
    print("This is must")


class MyException(Exception):          #从Exception继承错误，写自己的错误类

    def __init__(self,msg):
        self.error = msg

    def __str__(self, *args, **kwgs):  #__str__可以直接返回值
        return self.error

a = MyException('自定义错误信息')
print('{}'.format(a))                  #通过__str__用print直接打出错误信息


raise MyException('手动触发错误')      #主动触发异常