python中assertion异常

python中的assert是一种最简单的异常机制
assert的基本语法是：

"assert" expression1 ["," expression2]

expression1用于判断生成布尔值，当expression1为假的时候，则抛出异常，[]中的内容可选，即用户可以选择异常的提示值：

>>>a=23
>>>assert a==23
>>>a=a-1
>>>assert a==23
Traceback (most recent call last):
  File "", line 1, in 
AssertionError
>>>assert a==23 , "error1"
Traceback (most recent call last):
  File "", line 1, in 
AssertionError: error1