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