1. try & except

　　原程式：

1 import math
2 
3 while True:
4     text = raw_input('> ')
5     if text[0] == 'q':
6         break
7     x = float(text)
8     y = math.log10(x)
9     print "log10({0}) = {1}".format(x, y)

　　這段代碼接收命令行的輸入，當輸入為數字時，計算它的對數並輸出，直到輸入值為 q 為止。

　　但是當輸入0或者負數時，會報錯。

　　修改後的程式：

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 塊中的內容出現了異常，那麼 try 塊後面的內容會被忽略，Python會尋找 except 裡面有沒有對應的內容，如果找到，就執行對應的塊，沒有則拋出這個異常。

運行結果：

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

　　捕捉所有類型錯誤：

　　將except 的值改成 Exception 類，來捕獲所有的異常。

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"

　　運行結果：

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

　　進一步完善後的程式：

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"

　　運行結果：

> 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 塊有沒有異常， finally 塊的內容總是會被執行，而且會在拋出異常前執行，因此可以用來作為安全保證，比如確保打開的文件被關閉。

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

　　如果異常被捕獲了，在最後執行：

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

out: divide by 0.
     finally was called.