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")
谢谢
