python【第五篇】异常处理

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else

finally：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 "<stdin>", line 1, in <module>
NameError: xxx

 

>>> try:
...     raise NameError('xxx')
... except NameError, e:
...     print e
...
xxx

 

>>> import sys
>>> try:
...     10/0
... except:
...     res = sys.exc_info()
...
>>> print res
(<type 'exceptions.ZeroDivisionError'>, ZeroDivisionError('integer division or modulo by zero',), <traceback object at 0xb76eda7c>)

　　

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
class Sample:
    def __enter__(self):
        print("enter")
        # 获取资源
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        # 释放资源
        print("exit")

    def do_something(self):
        print("doing something")


with Sample() as sample:
    sample.do_something()

更简洁的用法

import contextlib

@contextlib.contextmanager
def file_open(file_name):
    print("file open")
    yield {}
    print("file end")

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