Python3基础之异常结构

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.