01Python基础_09异常

　　1. try & except

　　原程序：

import math

while True:
    text = raw_input('> ')
    if text[0] == 'q':
        break
    x = float(text)
    y = math.log10(x)
    print "log10({0}) = {1}".format(x, y)

　　这段代码接收命令行的输入，当输入为数字时，计算它的对数并输出，直到输入值为 q 为止。

　　但是当输入0或者负数时，会报错。

　　修改后的程序：

import math

while True:
    try:
        text = raw_input('> ')
        if text[0] == 'q':
            break
        x = float(text)
        y = math.log10(x)
        print "log10({0}) = {1}".format(x, y)
    except ValueError:
        print "the value must be greater than 0"

　　一旦 try 块中的内容出现了异常，那么 try 块后面的内容会被忽略，Python会寻找 except 里面有没有对应的内容，如果找到，就执行对应的块，没有则抛出这个异常。

运行结果：

> -1
the value must be greater than 0
> 0
the value must be greater than 0
> 1
log10(1.0) = 0.0
> q

　　捕捉所有类型错误：

　　将except 的值改成 Exception 类，来捕获所有的异常。

import math

while True:
    try:
        text = raw_input('> ')
        if text[0] == 'q':
            break
        x = float(text)
        y = 1 / math.log10(x)
        print "1 / log10({0}) = {1}".format(x, y)
    except Exception:
        print "invalid value"

　　运行结果：

> 1
invalid value
> 0
invalid value
> -1
invalid value
> 2
1 / log10(2.0) = 3.32192809489
> q

　　进一步完善后的程序：

import math

while True:
    try:
        text = raw_input('> ')
        if text[0] == 'q':
            break
        x = float(text)
        y = 1 / math.log10(x)
        print "1 / log10({0}) = {1}".format(x, y)
    except ValueError:
        print "the value must be greater than 0"
    except ZeroDivisionError:
        print "the value must not be 1"
    except Exception:
        print "unexpected error"

　　运行结果：

> 1
the value must not be 1
> -1
the value must be greater than 0
> 0
the value must be greater than 0
> q

　　2. finally

　　不管 try 块有没有异常， finally 块的内容总是会被执行，而且会在抛出异常前执行，因此可以用来作为安全保证，比如确保打开的文件被关闭。

try:
    print 1 / 0
finally:
    print 'finally was called.'

out: finally was called.

---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-13-87ecdf8b9265> in <module>()
      1 try:
----> 2     print 1 / 0
      3 finally:
      4     print 'finally was called.'

ZeroDivisionError: integer division or modulo by zero

　　如果异常被捕获了，在最后执行：

try:
    print 1 / 0
except ZeroDivisionError:
    print 'divide by 0.'
finally:
    print 'finally was called.'

out: divide by 0.
     finally was called.