错误

从软件方面来说,错误是语法或是逻辑上的

语法错误指示软件的结构上有错误,导致不能被解释器解释或编译器无法编译,这些错误必须在程序执行前纠正

逻辑错误可能是由于不完整或是不合法的输入所致

还可能是逻辑无法生成,计算,或是输出结果需要的过程无法执行

 

简单的语法错误

>>> KeyboardInterrupt>>> abcdTraceback (most recent call last):  File "
      
       ", line 1, in 
       
        NameError: name 'abcd' is not defined>>> pass = 'qbcd'  File "
        
         ", line 1    pass = 'qbcd'         ^SyntaxError: invalid syntax
        
       
      

异常

当python检测到一个错误时,解释器就会指出当前流已经无法继续执行下去,这时候出现了异常

异常是因为程序出现了错误而在正常控制流以外采取的行为

这个行为又分为两个阶段,首先是引起异常发生的错误,然后是检测（和采取可能的措施）阶段

当程序运行时,因为遇到了未解的错误而导致终止运行,便会出现traceback消息,打印异常

异常	描述
NameError	未声明/初始化对象
IndexError	序列中没有此索引
SyntaxError	语法错误
KeyboardInterruupt	用户中断执行
EOFError	没有内建输入,达到EOF标记
IOError	输入/输出操作失败

try-except语句

定义了进行异常监控的一段代码，并且提供了处理异常的机制

语法：

try

 try_suite#监控这里的异常

except Exception【，reason】

 except_suite#异常处理代码

>>> try:...     f = open('foo.txt')...   File "
        
         ", line 3        ^SyntaxError: invalid syntax>>> try:...     f = open('foo.txt')... except IOError:...     print "No such file"... No such file
        

#!/usr/bin/env pythonimport timefor i in range(1,11):    print i    try:        time.sleep(0.5)    except KeyboardInterrupt:        passprint 'done'[root@bogon untitled10]# ./Ptime.py 1234^C5^C6^C78910done

捕获所有异常

#!/usr/bin/env pythonnum = ''result = ''try:    num = int(raw_input("please input some number:>"))    result = 100/numexcept:    print "some error"print result[root@bogon untitled10]# ./Aerror.py please input some number:>0some error[root@bogon untitled10]# ./Aerror.py please input some number:>asome error[root@bogon untitled10]# ./Aerror.py please input some number:>^Csome error[root@bogon untitled10]# ./Aerror.py please input some number:>some error

改进代码

#!/usr/bin/env pythonnum = ''result = ''try:    num = int(raw_input("please input some number:>"))    result = 100/numexcept (KeyboardInterrupt,EOFError):    print "\nuser cancelled"except (ValueError,ZeroDivisionError),e:    print "\nError" ,eprint result#!/usr/bin/env pythonnum = ''result = ''try:    num = int(raw_input("please input some number:>"))    result = 100/numexcept (KeyboardInterrupt,EOFError):    print "\nuser cancelled"except (ValueError,ZeroDivisionError),e:    print "\nError" ,eprint result

raise,自定义异常

>>> raise aaaTraceback (most recent call last):  File "
        
         ", line 1, in 
         
          NameError: name 'aaa' is not defined>>> raise ValueErrorTraceback (most recent call last):  File "
          
           ", line 1, in 
           
            ValueError>>> raise ValueError ,'some error'Traceback (most recent call last):  File "
            
             ", line 1, in 
             
              ValueError: some error
             
            
           
          
         
        

#!/usr/bin/env pythondef set_age(name,age):    if age < 1 or age > 150:        raise ValueError,"age out of range"    print "%s is %s year old" % (name,age)set_age('bob',-1)/usr/bin/python2.6 /root/PycharmProjects/untitled10/Prasie.pyTraceback (most recent call last):  File "/root/PycharmProjects/untitled10/Prasie.py", line 8, in 
        
             set_age('bob',-1)  File "/root/PycharmProjects/untitled10/Prasie.py", line 5, in set_age    raise ValueError,"age out of range"ValueError: age out of rangeProcess finished with exit code 1
        

断言

断言是依据必须等价于布尔值为真的判定,此外发生异常也意味着表达式为假

#!/usr/bin/env pythondef set_age(name,age):    if age < 1 or age > 150:        raise ValueError,"age out of range"    print "%s is %s year old" % (name,age)def set_age2(name,age):    assert 0 < age <= 150,'Age out of range'    print "%s is %s year old" % (name,age)set_age('bob',20)set_age2('tom',200)/usr/bin/python2.6 /root/PycharmProjects/untitled10/Prasie.pybob is 20 year oldTraceback (most recent call last):  File "/root/PycharmProjects/untitled10/Prasie.py", line 13, in 
         
              set_age2('tom',200)  File "/root/PycharmProjects/untitled10/Prasie.py", line 9, in set_age2    assert 0 < age <= 150,'Age out of range'AssertionError: Age out of rangeProcess finished with exit code 1