【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!!