自定义异常类

class ShortInputException(Exception):

    def __init__(self, length, atleast):

        Exception.__init__(self)

        self.length = length

        self.atleast = atleast

try:

    s = input('Please Input --> ')

    if len(s)<3:

        raise ShortInputException(len(s), 3)

except EOFError:

    print('You input a end mark EOF')

except ShortInputException as x:

    print('ShortInputException: length is {0:,}, at least is {1:,}'.format(x.length, x.atleast))

else:

    print('no error and everything is ok.')

Please Input --> yuxingliangEOF

no error and everything is ok.

class MyError(Exception):

    def __init__(self, value):

        self.value = value

    def __str__(self):

        return repr(self.value)

try:

    raise MyError(2*2)

except MyError as e:

    print('My exception occurred, value: ', e.value)

My exception occurred, value:  4

0. 断言语句的应用

assert

作用：确认条件表达式是否满足，一般和异常处理结构一起使用。

结构：

assert 条件表达式, '表达式error时，给出此处的判定字符串提示。'

a,b = 3, 5

assert a == b, 'a must be equal to b.' #判定a是否等于b，if a ！= b，抛出异常

---------------------------------------------------------------------------

AssertionError                            Traceback (most recent call last)

<ipython-input-7-2ad85e920458> in <module>()

      1 a,b = 3, 5

----> 2 assert a == b, 'a must be equal to b.' #判定a是否等于b，if a ！= b，抛出异常

AssertionError: a must be equal to b.

try:

    assert a == b, 'a must be equal to b'

except AssertionError as reason:

    print('%s:%s'%(reason.__class__.__name__, reason))

AssertionError:a must be equal to b

1. try...except... 结构

• 如果try子句中的代码引发异常并被except子句捕捉，则执行except子句的代码块；
• 如果try子句中的代码块没有出现异常，则except子句代码块不执行，继续往后执行。

try:

    #可能会引发异常的代码，先执行以下试试看

except:

    #如果try中的代码抛出异常并被except捕捉，则执行此处的代码语句

"""代码功能：决策用户输入的是否是一个数字。

代码功能详细描述：while语句主导的死循环。

首先要求用户输入，然后就用户的输入进行判定：

尝试try中的语句

用户输入正确的数字，将输入的数字字符转换成数字，然后打印出提示信息，break掉循环

用户输入错误的字符，try中的语句检测到错误，然后被exception捕捉到，马上转到except中的语句执行，打印错误信息

虽有开始下一步的循环，知道用户输入正确的数字字符，采用break语句终止循环。"""

while True:

    x = input('Please input: ')

    try:

        x  = int(x)

        print('You have input {0}'.format(x))

        break

    except Exception as e:

        print('Error.')

Please input: a

Error.

Please input: 234f

Error.

Please input: 6

You have input 6

2. try...except...else...结构

try	except	else
检测语句	有问题，执行相应的处理代码	不执行else语句
检测语句	没问题，不执行except语句	执行else下的语句

try:

    #可能会引发错误的代码

except Exception as reason:

    #用来处理异常的代码

else:

    #如果try中的子句代码没有引发异常，就执行此处的代码

while True:

    x = input('Please input: ')

    try:

        x  = int(x)  # 此处是可能引发异常的语句

    except Exception as e:

        print('Error.') # 处理异常的语句

    else:               # 没有异常时，处理的语句

        print('You have input {0}'.format(x))

        break

Please input: a

Error.

Please input: b

Error.

Please input: 664h

Error.

Please input: 666

You have input 666

3. try...except...finally...结构

try	except	finally
尝试语句	有问题，执行相应的处理代码	始终执行finally语句
尝试语句	没问题，不执行except语句	始终执行finally语句

try:

    #可能会引发错误的代码

except Exception as reason:

    #用来处理异常的代码

finally:

    #不论try中是否引发异常，始终执行此处的代码

def div(a,b):

    try:

        print(a/b)

    except ZeroDivisionError:

        print('The second parameter cannot be 0.')

    finally:

        print(-1)

div(3,5)

0.6

-1

div(3,0)

The second parameter cannot be 0.

-1

如果try子句中的异常没有被except语句捕捉和处理，或者except子句或者else子句中的代码抛出的了异常，

那么这些异常将会在finally子句执行完毕之后再次抛出异常。

div('3',5)

-1

---------------------------------------------------------------------------

TypeError                                 Traceback (most recent call last)

<ipython-input-15-dc6751a7464e> in <module>()

----> 1 div('3',5)

<ipython-input-12-d5530669db53> in div(a, b)

      1 def div(a,b):

      2     try:

----> 3         print(a/b)

      4     except ZeroDivisionError:

      5         print('The second parameter cannot be 0.')

TypeError: unsupported operand type(s) for /: 'str' and 'int'

4. 捕捉多种异常的结构

try:

    #可能会引发异常的代码

except Exception1:

    #处理异常类型1的代码

except Exception2:

    #处理异常类型2的代码

except Exception3:

    #处理异常类型3的代码

.

.

.

try:

    x = float(input('Please the first number: '))

    y = float(input('Please the second number: '))

    z = x/y

except ZeroDivisionError:

    print('the second number isnot 0.')

except TypeError:

    print('the number must be number.')

except NameError:

    print('The variable isnot here.')

else:

    print(x, '/', y, '=', z)

Please the first number: 30

Please the second number: 5

30.0 / 5.0 = 6.0

try:

    x = float(input('Please the first number: '))

    y = float(input('Please the second number: '))

    z = x/y

except (ZeroDivisionError, TypeError, NameError):

    print('Error is catched.')

else:

    print(x, '/', y, '=', z)

Please the first number: 45

Please the second number: 0

Error is catched.

5. 多种结构混合

def div(x,y):

    try:

        print(x/y)

    except ZeroDivisionError:

        print('ZeroDivisionError')

    except TypeError:

        print('typeerror')

    else:

        print('no error')

    finally:

        print('I am executing finally clause.')

div(3,4)

0.75

no error

I am executing finally clause.