Python 语法之例外

Python 语法之例外

File information

2009-10-24

磁针石：xurongzhong#gmail.com

博客：oychw.cublog.cn

参考资料：

《Python Essential Reference 4th Edition 2009》

《beginning python from novice to professional second edition 2008》

 

 

       产生例外：raise Exception，raise Exception('hyperdrive overload')，raise RuntimeError("Unrecoverable Error")

       查看例外种类： import exceptions 然后再：dir(exceptions),可以直接调用这些例外种类，比如：raise ArithmeticError。

       重要的例外种类：Exception、AttributeError、IOError、IndexError、KeyError、NameError、SyntaxError、TypeError、ValueError、ZeroDivisionError

       如果需要没有捕捉到的例外，可以送到sys.excepthook()

       内置例外见参考1：87页

 

       自定义例外类：

              class SomeCustomException(Exception): pass

 

              class DeviceError(Exception):

              def __init__(self,errno,msg):

              self.args = (errno, msg)

              self.errno = errno

              self.errmsg = msg

              # Raises an exception (multiple arguments)

              raise DeviceError(1, 'Not Responding')

        获取例外

              try:

              x = input('Enter the first number: ')

              y = input('Enter the second number: ')

              print x/y

              except ZeroDivisionError:

              print "The second number can't be zero!"

 

       也可以用if语句来判断，不过如果有多个分母，一个try就可以搞定了。

       在没有交互式的情况，使用print更好,比如记录在日志中：

              class MuffledCalculator:

              muffled = False

              def calc(self, expr):

              try:

              return eval(expr)

              except ZeroDivisionError:

              if self.muffled:

              print 'Division by zero is illegal'

              else:

              raise

 

       同时捕捉多个错误，上面的例子中，如果输入数字改用字符串则会报类型不匹配

              try:

              x = input('Enter the first number: ')

              y = input('Enter the second number: ')

              print x/y

              except ZeroDivisionError:

              print "The second number can't be zero!"

              except TypeError:

              print "That wasn't a number, was it?"

 

       多个例外也可以放在一个子句:

              try:

              x = input('Enter the first number: ')

              y = input('Enter the second number: ')

              print x/y

              except (ZeroDivisionError, TypeError, NameError):

              print 'Your numbers were bogus...'

 

       捕捉对象，比如用log记录例外：

              try:

              x = input('Enter the first number: ')

              y = input('Enter the second number: ')

              print x/y

              except (ZeroDivisionError, TypeError) as e:

              print e

 

              try:

              do something

              except:

              error_log.write('An error occurred\n')

 

 

       捕捉所有错误：

              try:

              x = input('Enter the first number: ')

              y = input('Enter the second number: ')

              print x/y

              except:

              print 'Something wrong happened...'

       一般不建议使用，可能会隐藏错误。

       try也可以跟else：

 

              try:

              f = open('foo', 'r')

              except IOError as e:

              error_log.write('Unable to open foo : %s\n' % e)

              else:

              data = f.read()

              f.close()

 

       修改上面程序直至输入正确：

             

              while True:

              try:

              x = input('Enter the first number: ')

              y = input('Enter the second number: ')

              value = x/y

              print 'x/y is', value

              except Exception, e:

              print 'Invalid input:', e

              print 'Please try again'

              else:

              break

                    

 

       最后还有finally可以用来收尾，比如关闭文件等。

 

              try:

              1/0

              except NameError:

              print "Unknown variable"

              else:

              print "That went well!"

              finally:

              print "Cleaning up."

      

       例外不处理会累积至上级调用，直至处理为止。到最高层还不处理，会导致运行停止。

 

       普通语句处理与例外处理：

              if语句

              def describePerson(person):

              print 'Description of', person['name']

              print 'Age:', person['age']

              if 'occupation' in person:

              print 'Occupation:', person['occupation']

             

              例外：

              def describePerson(person):

              print 'Description of', person['name']

              print 'Age:', person['age']

              try:

              print 'Occupation: ' + person['occupation']

              except KeyError: pass

       可以if语句处理要先检查是否有这个元素，再读取。例外处理有效一点，但不会特别明显。try/except和if/else有很大的相似性。