作用

程序遇到异常会退出，处理异常增加程序的健壮性

常见异常

NameError 尝试访问一个没有声明的变量

ImportError 无法引入模块或包；可能路径不存在

IndentationError 语法错误(的子类)；代码没有正确对齐

SyntaxError 语法错误

IndexError 索引超出序列范围

KeyError 请求一个不存在的字典关键字

IOError 输入输出错误(比如你要读的文件不存在)

AttributeError 尝试访问未知的对象属性

ValueError 传给函数的参数类型不正确，比如给int()函数传入字符串类型

UnboundLocalError 试图访问一个还未被设置的局部变量

KeyboardInterrupt

语法

异常在try块里抛。

try有异常，执行 except ；try无异常，才会执行elsefinally：无论try块是否抛异常，永远执行的代码，通常用来执行关闭文件，断开服务器连接等功能。

例子

try代码块有错误则执行except代码块

except m,n:m为异常名称，e为变量，异常信息赋值给它

>>> try:

... r=10/0

... print('result is %f' % r)

... except ZeroDivisionError,e:

... print('ZeroDivisionError: %s' %e)

...

ZeroDivisionError: integer division or modulo by zero

一下代码均放在exception.py文件中

一旦捕获到即跳出try代码块，跳到对应的exception语句，except类似于if

try:

r=10/0

print('result: %f' % r)

except ZeroDivisionError, e:

print('%s' % e)

except NameError, e:

print('%s' % e)

print('end')

[root@hy tmp]# python exception.py

integer division or modulo by zero

end

try没有异常则跳到else语句

try:

r=10/0

print('result: %f' % r)

except ZeroDivisionError, e:

print('%s' % e)

except NameError, e:

print('%s' % e)

else:

print('no exceptions')

print('end')

[root@hy tmp]# python exception.py

integer division or modulo by zero

end

try:

r=10/1

print('result: %f' % r)

except ZeroDivisionError, e:

print('%s' % e)

except NameError, e:

print('%s' % e)

else:

print('no exceptions')

print('end')

[root@hy tmp]# python exception.py

result: 10.000000

no exceptions

end

try:

r=10/1

print aaa

print('result: %f' % r)

except ZeroDivisionError, e:

print('%s' % e)

except NameError, e:

print('result: %s' % e)

else:

print('no exceotions')

print('end')

[root@hy tmp]# python exception.py

result: name 'aaa' is not defined

end

无论try有没有捕获到异常，finally语句都会执行

try:

r=10/1

print aaa

print('result: %f' % r)

except ZeroDivisionError, e:

print('%s' % e)

except NameError, e:

print('result: %s' % e)

else:

print('no exceotions')

finally:

print('finally')

print('end')

[root@hy tmp]# python exception.py

result: name 'aaa' is not defined

finally

end

父类异常在前则只执行父类异常

try:

r=10/0

print('result: %f' % r)

except StandardError, e:

print('StandardError')

except ZeroDivisionError, e:

print('%s' % e)

else:

print('no exceotions')

finally:

print('finally')

print('end')

[root@hy tmp]# python exception.py

StandardError

finally

end

try:

r=10/0

print('result: %f' % r)

except ZeroDivisionError, e:

print('%s' % e)

except StandardError, e:

print('StandardError')

else:

print('no exceotions')

finally:

print('finally')

print('end')

[root@hy tmp]# python exception.py

integer division or modulo by zero

finally

end

raise自定义异常

>>> raise NameError('xxx')

Traceback (most recent call last):

File "", line 1, in

NameError: xxx

>>> try:

... raise NameError('xxx')

... except NameError, e:

... print e

...

xxx

>>> import sys

>>> try:

... 10/0

... except:

... res = sys.exc_info()

...

>>> print res

(, ZeroDivisionError('integer division or modulo by zero',), )

class CustomError(Exception):

def __init__(self,ErrorInfo):

super().__init__(self) #初始化父类

self.errorinfo=ErrorInfo

def __str__(self):

return self.errorinfo

if __name__ == '__main__':

try:

raise CustomError('客户异常')

except CustomError as e:

print(e)

class DatabaseException(Exception):

def __init__(self,err='数据库错误'):

Exception.__init__(self,err)

class PreconditionsException(DatabaseException):

def __init__(self,err='PreconditionsErr'):

DatabaseException.__init__(self,err)

def testRaise():

raise PreconditionsException()

try:

testRaise()

except PreconditionsException as e:

print (e)

上下文管理器

作用：简化释放资源时的异常语法

#上下文管理器协议

classSample:def __enter__(self):print("enter")#获取资源

returnselfdef __exit__(self, exc_type, exc_val, exc_tb):#释放资源

print("exit")defdo_something(self):print("doing something")

with Sample() as sample:

sample.do_something()

更简洁的用法

importcontextlib

@contextlib.contextmanagerdeffile_open(file_name):print("file open")yield{}print("file end")

with file_open("bobby.txt") as f_opened:print("file processing")