Python-异常处理

一、捕获异常

1、当程序出错时，不会展示bug信息给用户，而是提供一个友好的提示后，继续执行程序

2、如下，实现除法，当分母为0时，捕获并打印出异常信息；Exception可以捕获任何异常

def div(x, y):
    try:
        print(x/y)
    except Exception as e:
        print('异常为', e)

>>> div(1,0)
异常为 division by zero

也可以指定异常为ZeroDivisionError

def div(x, y):
    try:
        print(x/y)
    except ZeroDivisionError as e:
        print('异常为', e)

>>> div(1,0)
异常为 division by zero

3、同时捕获多个异常

def fdiv(x,y):
    try:
        return x/y
    except TypeError as e:
        print('TypeError: ', e)
    except ZeroDivisionError as e:
        print('ZeroDivisionError: ', e)
    except Exception as e:
        print('其他异常：', e)

>>> fdiv('ABC', 0)
TypeError: unsupported operand type(s) for /: 'str' and 'int'
>>> fdiv(12,0)
ZeroDivisionError: division by zero

也可以用以下方式：

def fdiv1(x,y):
    try:
        return x/y
    except (TypeError, ZeroDivisionError, Exception) as e:
        print(e)


>>> fdiv1('ABC', 0)
unsupported operand type(s) for /: 'str' and 'int'
>>> fdiv1(12,0)
division by zero

二、常用异常

'---------------IOError:打开文件路径出错或不存在------------------'
try:
    f = open('error.txt','r')
except IOError as e:
    print('打开文件出错, 错误信息：', e)

'--------------ImportError：导入模块路径出错或不存在--------------'
try:
    import error
except ImportError as e:
    print('导入模块出错， 错误信息：', e)

'--------------IndexError：索引出界----------------------------'
list1 = [1,2,3,4,5,6]
try:
    print(list1[6])
except IndexError as e:
    print('查询列表出错， 错误信息：', e)

'----------------keyError：查找字典中不存在的关键字---------------'
dict1 = {1:'A', 2:'B'}
try:
    print(dict1[3])
except KeyError as e:
    print('查询字典出错， 错误信息：', e)

'----------------TypeError：参数类型不匹配----------------------'
def fint(a):
    print(a/2)
try:
    str1 = 'ABC'
    fint(str1)
except TypeError as e:
    print('参数类型出错， 错误信息：', e)

'------------ValueError：参数类型匹配，但是传入的值不对------------'
str1 = 'ABC'
try:
    print(int(str1))
except ValueError as e:
    print('参数值出错， 错误信息：', e)

'------------------NameError:使用的变量没有赋值------------------'
try:
    print(a)
except NameError as e:
    print('参数出错， 错误信息：', e)

三、自定义异常

class MyException(Exception):
    def __init__(self, msg):
        self.message = msg

    def __str__(self):
        return self.message

try:
    raise MyException('自定义异常')
except MyException as e:
    print(e)

四、主动抛出异常

try:
    raise IOError('读写文件出错了')  #主动抛出IOError异常
except IOError as e:
    print(e)

五、try - except - else - finally

try:
    f = open('test.txt', 'r')
except IOError as e:  #打开文件失败，则抛出异常
    print(e)
else:                 #打开文件成功，则执行else块
    print('执行else')
    f.close()
finally:              #无论文件是否正常打开，都会执行finally块
    print('执行finally')