处理多个异常
处理多个异常,并不是同时报出多个异常。程序运行时,只要遇到一个异常,就会有所反应。所以,每次捕获到的异常,一定是一个。所以,所谓的处理多个异常,意思是说容许捕获不同的异常,在不同的except子句中处理。
形式如下:
try:
print "try子句"
except xxx:
print "异常xxx"
except yyy:
print "异常yyy"
例子:
#!usr/bin/env Python
# coding=utf-8
while 1:
print "this is a division program."
c = raw_input("input 'c' continue,otherwise logot:")
if c == 'c':
a = raw_input("first number:")
b = raw_input("second number:")
try:
print float(a)/float(b)
except ZeroDivisionError:
print "the second number can't be zero"
print "************************"
except ValueError:
print "please input number"
print "************************"
else:
break
与上一篇的例子相比,多了一个异常捕获。当输入的分母不是数字时,可以捕获该异常。测试如下:
$ python tryExcept1.py
this is a division program.
input 'c' continue,otherwise logot:c
first number:3
second number:a
please input number
************************
this is a division program.
input 'c' continue,otherwise logot:c
first number:3
second number:0
the second number can't be zero
************************
this is a division program.
input 'c' continue,otherwise logot:c
first number:5
second number:2
2.5
this is a division program.
input 'c' continue,otherwise logot:p
$
除了使用多个except之外,还可以在一个except后面放多个异常参数,可以将except部分修改为如下:
except (ZeroDivisionError,ValueError):
print "please input rightly"
print "*********************"
运行结果:
$ python tryExcept1.py
this is a division program.
input 'c' continue,otherwise logot:c
first number:2
second number:a
please input rightly
*********************
this is a division program.
input 'c' continue,otherwise logot:c
first number:3
second number:0
please input rightly
*********************
this is a division program.
input 'c' continue,otherwise logot:c
first number:4
second number:2
2.0
this is a division program.
input 'c' continue,otherwise logot:d
mobao:PythonExample mobao$
特别注意,一个except,多个异常参数时,异常参数需要放进()中。
如果需要呈现内置的异常信息,则可做如下更改:
except (ZeroDivisionError,ValueError), e:
print e
print "*********************"
在Python3中,则如下:
except (ZeroDivisionError,ValueError) as e:
print e
print "*********************"
运行效果:
python tryExcept1.py
this is a division program.
input 'c' continue,otherwise logot:c
first number:2
second number:a
could not convert string to float: a
*********************
this is a division program.
input 'c' continue,otherwise logot:c
first number:2
second number:0
float division by zero
*********************
this is a division program.
"could not convert string to float: a"和"float division by zero"皆为内置异常信息。
以上的写法,处理了两个异常,如果是不止两个异常呢?可以这样:execpt: 或者 except Exception, e,后面什么参数也不写就好了。
else子句
一般try...except...,在一般情况下是够用的。但是总有不一般的时候出现,所以增加了一个else子句。如:
>>> try:
... print "I am try"
... except:
... print "I am except"
... else:
... print "I am else"
...
I am try
I am else
可以看出,如果执行了try,则except()子句会被直接忽略,else子句会被执行。
>>> try:
... print 1/0
... except:
... print "I am except"
... else:
... print "I am else"
...
I am except
如果执行了except子句,则不执行else中的文件。
例如:
#!usr/bin/env Python
# coding=utf-8
while 1:
try:
a = raw_input("first number:")
b = raw_input("second number:")
print float(a)/float(b)
except Exception, e:
print e
print "try again"
else:
break
结果如下:
$ python tryExcept1.py
first number:2
second number:a
could not convert string to float: a
try again
first number:3
second number:0
float division by zero
try again
first number:5
second number:2
2.5
$
finally
无论try子句执行还是except子句执行,finally中一定执行。
>>> x = 10
>>> try:
... x = 2/1
... except Exception,e:
... print e
... finally:
... print "del x"
... del x
...
del x
>>> x = 10
>>> try:
... x = 1/0
... except Exception,e:
... print e
... finally:
... print "del x"
... del x
...
integer division or modulo by zero
del x
当然,在应用中,可以将上面的各个子句都综合起来使用,写成如下样式:
try:
do something
except:
do something
else:
do something
finally
do something
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