Python-異常處理

一、捕獲異常

1、當程序出錯時，不會展示bug信息給用戶，而是提供一個友好的提示后，繼續執行程序

2、如下，實現除法，當分母為0時，捕獲並打印出異常信息；Exception可以捕獲任何異常

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

>>> div(1,0)
異常為 division by zero

 

也可以指定異常為ZeroDivisionError

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

>>> div(1,0)
異常為 division by zero

 

3、同時捕獲多個異常

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('其他異常：', 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、常用異常

'---------------IOError:打開文件路徑出錯或不存在------------------'
try:
    f = open('error.txt','r')
except IOError as e:
    print('打開文件出錯, 錯誤信息：', e)

'--------------ImportError：導入模塊路徑出錯或不存在--------------'
try:
    import error
except ImportError as e:
    print('導入模塊出錯， 錯誤信息：', e)

'--------------IndexError：索引出界----------------------------'
list1 = [1,2,3,4,5,6]
try:
    print(list1[6])
except IndexError as e:
    print('查詢列表出錯， 錯誤信息：', e)

'----------------keyError：查找字典中不存在的關鍵字---------------'
dict1 = {1:'A', 2:'B'}
try:
    print(dict1[3])
except KeyError as e:
    print('查詢字典出錯， 錯誤信息：', e)

'----------------TypeError：參數類型不匹配----------------------'
def fint(a):
    print(a/2)
try:
    str1 = 'ABC'
    fint(str1)
except TypeError as e:
    print('參數類型出錯， 錯誤信息：', e)

'------------ValueError：參數類型匹配，但是傳入的值不對------------'
str1 = 'ABC'
try:
    print(int(str1))
except ValueError as e:
    print('參數值出錯， 錯誤信息：', e)

'------------------NameError:使用的變量沒有賦值------------------'
try:
    print(a)
except NameError as e:
    print('參數出錯， 錯誤信息：', e)

 

三、自定義異常

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

    def __str__(self):
        return self.message

try:
    raise MyException('自定義異常')
except MyException as e:
    print(e)

 

四、主動拋出異常

try:
    raise IOError('讀寫文件出錯了')  #主動拋出IOError異常
except IOError as e:
    print(e)

 

五、try - except - else - finally

try:
    f = open('test.txt', 'r')
except IOError as e:  #打開文件失敗，則拋出異常
    print(e)
else:                 #打開文件成功，則執行else塊
    print('執行else')
    f.close()
finally:              #無論文件是否正常打開，都會執行finally塊
    print('執行finally')