【脚本语言系列】关于Python异常处理，你需要知道的事

如何进行异常处理

Python异常类定义在exceptions模块中，并继承自基类BaseException.
BaseException类有三个子类，为Exception,KeyboardInterrupt,SystemExit.
Exception类是异常类，包括StandardError,StopIteration,GeneratorExit,Warning.
try…except语句用于处理问题语言，捕获可能出现的异常。

try…except

# -*- coding:utf-8 -*-# catch IOErrortry:    file("hello.txt","r")    print "read file"except IOError:    print "File Not Exit"except:    print "program abnormal"

read file

# -*- coding:utf-8 -*-# catch ZeroDivisionErrortry:    result = 10/0except ZeroDivisionError:    print "0 Division Error"

0 Division Error

# -*- coding:utf-8 -*-# catch ZeroDivisionErrortry:    result = 10/0except ZeroDivisionError:    print "0 Division Error"else:    print result

0 Division Error

# -*- coding:utf-8 -*-# nest the Errortry:    s = "hello"    try:        print s[0]+s[1]        print s[0]-s[1]    except TypeError:        print "String not support -"except:    print "Exception"

heString not support -

try…finally

# -*- coding:utf-8 -*-# try the finallytry:    f = open("hello.txt","r")    print "read file"except IOError:    print "file not exist"finally:    f.close()

read file

# -*- coding:utf-8 -*-# nest the finallytry:    f = open("hello.txt","r")    try:        print f.read(5)    except:        print "read file Error"    finally:        print "release the src"        f.close()except IOError:    print "file not exist"

read file

raise

# -*- coding:utf-8 -*-# try the raisetry:     s = None    if s is None:        print "s is empty object"        raise NameError    print len(s)except TypeError:    print "Empty object has no length"

s is empty object---------------------------------------------------------------------------NameError                                 Traceback (most recent call last)<ipython-input-10-084f89ed5ff6> in <module>()      5     if s is None:      6         print "s is empty object"----> 7         raise NameError      8     print len(s)      9 except TypeError:NameError: 

自定义异常

#  -*- coding:utf-8 -*-# self the exceptionfrom __future__ import divisionclass DivisionException(Exception):    def __init__(self, x, y):        Exception.__init__(self, x, y)        self.x = x        self.y = yif __name__ == "__main__":    try:        x = 7        y = 5        if x % y > 0:            print x/y            raise DivisionException(x, y)    except DivisionException, div:        print "DivisionException: x / y = %.2f" %(div.x/div.y)

1.4DivisionException: x / y = 1.40

assert

# -*- coding:utf-8 -*-# use the assertt = ("hello",)assert len(t) >= 1t = ("hello")assert len(t) == 1

---------------------------------------------------------------------------AssertionError                            Traceback (most recent call last)<ipython-input-13-e1aea18859ed> in <module>()      4 assert len(t) >= 1      5 t = ("hello")----> 6 assert len(t) == 1AssertionError: 

# -*- coding:utf-8 -*-# use the assert with msgmonth = 13assert 1 <= month <= 12, "month errors"

---------------------------------------------------------------------------AssertionError                            Traceback (most recent call last)<ipython-input-21-4aeda76e0fa4> in <module>()      2 # use the assert with msg      3 month = 13----> 4 assert 1 <= month <= 12, "month errors"AssertionError: month errors

exception 异常信息

# -*- coding:utf-8 -*-def fun():    a = 10    b = 0    return a / bdef format():    print "a / b = " + str(fun())if __name__ == "__main__":    format()

---------------------------------------------------------------------------ZeroDivisionError                         Traceback (most recent call last)<ipython-input-20-1916b966487f> in <module>()      9      10 if __name__ == "__main__":---> 11     format()<ipython-input-20-1916b966487f> in format()      6       7 def format():----> 8     print "a / b = " + str(fun())      9      10 if __name__ == "__main__":<ipython-input-20-1916b966487f> in fun()      3     a = 10      4     b = 0----> 5     return a / b      6       7 def format():ZeroDivisionError: division by zero

# -*- coding:utf-8 -*-import systry:    x = 10 / 0except Exception, ex:    print exprint sys.exc_info()

division by zero(None, None, None)

什么是异常处理

异常是程序中的例外、违例情况。
异常机制是指当程序出现错误后，程序的处理方法，
异常机制提供了程序正常退出的安全通道。
当出现错误后，程序执行的流程发生改变，程序的控制权转移到异常处理器。
当异常被引发时，如果没有代码处理该异常，异常将被Python接受处理。
当异常发生时，Python解释器将输出一些相关的信息并终止程序的运行。

为何进行异常处理

通过捕获异常可以提高程序的健壮性。
异常处理还具有释放对象、中止循环的运行等作用。