1. try & except
原程序：
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)
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.
