异常处理
1、什么是异常
异常是错误发生的信号
程序一旦出错，如果程序中还没有相应处理的机制
那么该错误就会产生一个异常抛出来
程序的运行也随之终止
2、一个异常分为三部分：
1、异常的追踪信息
2、异常的类型
3、异常的值
3、异常的分类：
1、语法异常：
这类异常应该在程序执行前就改正
print('start......') x=1 y x+=1 if print('stop......')
2、逻辑上的异常
IndexError # l=['a','b'] # l[100] KeyError # d={'a':1} # d['b'] AttributeError: # class Foo: # pass # # Foo.x # import os # os.aaa ZeroDivisionError # 1 / 0
FileNotFoundError # f=open('a.txt','r',encoding='utf-8') # ValueError: I/O operation on closed file. # f=open('a.txt','r',encoding='utf-8') # f.close() # f.readline() #ValueError: invalid literal for int() with base 10: 'aaaaa' # int('aaaaa') # TypeError # for i in 333: # pass #NameError # x # func()
def func() import os os.xxxx func()
单分支
try: # print('start.....') # x=1 # y # l=[] # l[3] # d={'a':1} # d['b'] # print('end....') # except NameError: # print('变量名没有定义') # # # print('other.....')
多分支
try: # print('start.....') # x=1 # # y # l=[] # l[3] # d={'a':1} # d['b'] # print('end....') # except NameError: # print('变量名没有定义') # except KeyError: # print('字典的key不存在') # except IndexError: # print('索引超出列表的范围') # # # print('other.....')
多种异常采用同一段逻辑处理
try: # print('start.....') # x=1 # # y # l=[] # # l[3] # d={'a':1} # d['b'] # print('end....') # except (NameError,KeyError,IndexError): # print('变量名或者字典的key或者列表的索引有问题') # print('other.....')
万能异常
try: # print('start.....') # x=1 # # y # l=[] # # l[3] # d={'a':1} # # d['b'] # import os # os.aaa # print('end....') # except Exception: # print('万能异常---》') # # # print('other.....')
获取异常的值
try: # print('start.....') # x=1 # y # l=[] # l[3] # d={'a':1} # d['b'] # import os # os.aaa # print('end....') # except Exception as e: # print('万能异常---》',e) # # # print('other.....')
try.......else......
else:不能单独使用，必须与except连用，意识是：else的字代码块会在被检测的代码没有
出现过任何异常的情况下执行
try: # print('start.....') # # x=1 # # # y # # l=[] # # l[3] # # d={'a':1} # # d['b'] # # import os # # os.aaa # print('end....') # except NameError as e: # print('NameError: ',e) # # except KeyError as e: # print('KeyError: ',e) # # except Exception as e: # print('万能异常---》',e) # # else: # print('在被检测的代码块没有出现任何异常的情况下执行') # print('other.....')
try.....finally....
try: # print('start.....') # # x=1 # # # y # # l=[] # # l[3] # # d={'a':1} # # d['b'] # # import os # # os.aaa # print('end....') # except NameError as e: # print('NameError: ',e) # # except KeyError as e: # print('KeyError: ',e) # # except Exception as e: # print('万能异常---》',e) # # else: # print('在被检测的代码块没有出现任何异常的情况下执行') # finally: # print('无论有没有异常发生，都会执行') # print('other.....')
finally的子代码块中通常放回收系统资源的代码
try: # f=open('a.txt',mode='w',encoding='utf-8') # f.readline() # # finally: # f.close() #这样确保能把文件关闭，不用担心因为前面报错，导致不会运行到关闭文件这一步。 # # print('other....')
主动触发异常
raise TypeError('类型错误') # class People: # def __init__(self,name): # if not isinstance(name,str): # raise TypeError('%s 必须是str类型' %name) # # self.name=name # # p=People(123)
断言
print('part1........') stus=['egon','alex','wxx','lxx'] stus=[] if len(stus) <= 0: raise TypeError #这上下两步其实是相同的，下面的意思是断定必须是要大于0才能运行下一步，小于0报错 assert len(stus) > 0 print('part2.........') print('part2.........') print('part2.........') print('part2.........') print('part2.........') print('part2.........')
自定义异常
class RegisterError(BaseException): def __init__(self,msg,user): self.msg=msg self.user=user def __str__(self): return '<%s：%s>' %(self.user,self.msg) raise RegisterError('注册失败','teacher')