简明Python教程学习笔记9
13、异常
（1）错误
程序中的一些无效语句，比如语法错误，如下所示：
1 >>> Print "aa" 2 SyntaxError: invalid syntax 3 >>> print "aa" 4 aa 5 >>>
（2）try...except
1 >>> s = raw_input("Enter something -->") 2 Enter something -->#这里期望输入，但是实际执行了ctrl+d，导致了下面的异常 3 4 Traceback (most recent call last): 5 File "<pyshell#4>", line 1, in <module> 6 s = raw_input("Enter something -->") 7 EOFError: EOF when reading a line 8 >>>
可以使用try...except来处理异常
1 # -*- coding:utf-8 -*- 2 3 import sys 4 5 try: 6 s = raw_input("Enter something -->") 7 except EOFError: 8 print "\nWhy did yo do an EOF on me?" 9 sys.exit() # Exit the program 10 except: 11 print "\nSome error/exception occurred." 12 # here, we are not exiting the program 13 14 print "Done"
输出：
（3）引发异常
可以使用raise语句 引发 异常。
还得指明错误/异常的名称和伴随异常 触发的 异常对象。
可以引发的错误或异常应该分别是一个Error或Exception类的直接或间接导出类。
1 # -*- coding:utf-8 -*- 2 3 4 class ShortInputException(Exception): 5 """A user-defined exception class.""" 6 7 def __init__(self, length, atleast): 8 Exception.__init__(self) 9 self.length = length 10 self.atleast = atleast 11 12 13 try: 14 s = raw_input("Enter something -->") 15 if len(s) < 3: 16 raise ShortInputException(len(s), 3) 17 # other work can continue as usual here 18 except EOFError: 19 print "\nWhy did you do an EOF on me?" 20 except ShortInputException, x: 21 print "ShortInputException:The input was of length %d,was expecting at least %d" % (x.length, x.atleast) 22 else: 23 print "No exception was raised."
输出：
（4）try...finally
希望在无论异常发生与否的情况下都执行某些操作，该怎么做呢？
这可以使用finally块来完成。
