【python】入门学习（八）

异常处理：

python在遇到问题时会自动引发异常，也可以用raise故意引发异常，异常种类必须是已有的

>>> raise IOError('This is a test.')

Traceback (most recent call last):

  File "<pyshell#21>", line 1, in <module>

    raise IOError('This is a test.')

OSError: This is a test.

IOError:打不开相应文件

ZeroDivisionError:除以0

SyntaxError:语法错误

 

捕获异常：

try/expect

def get_age():

    while True:

        try:

            n = int(input('How old are you?'))

            return n

        except ValueError:

            print('please enter an integer value.')

>>> get_age()

How old are you?a

please enter an integer value.

How old are you?-2

-2

捕获多种异常：把多个异常种类括起来就好了

def convert_to_int1(s,base):

    try:

        return int(s,base)

    except (ValueError, TypeError):

        return 'error'

>>> convert_to_int1(100,5)

'error'

>>> convert_to_int1(5,100)

'error'

>>> convert_to_int1('100',5)

25

分别处理不同的异常：用多个except

def convert_to_int2(s,base):

    try:

        return int(s,base)

    except ValueError:

        return 'value error'

    except TypeError:

        return 'type error'

>>> convert_to_int2(100,5)

'type error'

>>> convert_to_int2('100',1)

'value error'

捕获所有异常：except后面不加异常类型

def convert_to_int3(s,base):

    try:

        return int(s,base)

    except:

        return 'error'

>>> convert_to_int3('100',5)

25

 

清理操作：finally 无论是否引发异常都会执行

def invert(x):

    try:

        return 1/x

    except:

        return 'error'

    finally:

        print('invert(%s) done' %x)

>>> invert(2)

invert(2) done

0.5

>>> invert(0)

invert(0) done

'error'

 

with 尽早执行清理操作

下面代码将在for循环后立刻关闭文件

def add_linenum(fname):

    num = 1

    with open(fname,'r') as f:

        for line in f:

            print('%04d %s' %(num,line), end ='')

            num = num+1

>>> add_linenum('story.txt')

0001      Story    

0002 

0003 Mary had a litte lamb,

0004 and then she had some more.HaHaHa!!