Python----面向对象---异常处理

一、什么是异常

异常是错误发生的信号，一旦程序出错，并且程序没有处理这个错误，那么就会抛出异常，并且程序的运行随之终止

例如：

print('1')
print('2')
print('3')
int('aaaa')
print('4')
print('5')
print('6')

结果为：

Traceback (most recent call last):
1
2
  File "C:/Users/xu516/PycharmProjects/Python全栈开发/第三模块/面向对象编程/32 异常处理.py", line 6, in <module>
3
    int('aaaa')
ValueError: invalid literal for int() with base 10: 'aaaa'

报错后后面的代码没有被执行

二、错误分为两种

1、语法错误

例如：

print(xxx

if 1 > 3

结果为：

  File "C:/Users/xu516/PycharmProjects/Python全栈开发/第三模块/面向对象编程/32 异常处理.py", line 15
    
    ^
SyntaxError: unexpected EOF while parsing

2、逻辑错误

 a、ValueError

int('aaa')

结果为：

Traceback (most recent call last):
  File "C:/Users/xu516/PycharmProjects/Python全栈开发/第三模块/面向对象编程/32 异常处理.py", line 15, in <module>
    int('aaa')
ValueError: invalid literal for int() with base 10: 'aaa'

b、NameError

name

结果为：

Traceback (most recent call last):
  File "C:/Users/xu516/PycharmProjects/Python全栈开发/第三模块/面向对象编程/32 异常处理.py", line 17, in <module>
    name
NameError: name 'name' is not defined

c、IndexError

l = [1, 2, 3]
l[100]

结果为：

Traceback (most recent call last):
  File "C:/Users/xu516/PycharmProjects/Python全栈开发/第三模块/面向对象编程/32 异常处理.py", line 20, in <module>
    l[100]
IndexError: list index out of range

d、KeyError

d = {}
d['name']

结果为：

Traceback (most recent call last):
  File "C:/Users/xu516/PycharmProjects/Python全栈开发/第三模块/面向对象编程/32 异常处理.py", line 23, in <module>
    d['name']
KeyError: 'name'

e、AttributeError

class Foo:
    pass

Foo.xxx

结果为：

Traceback (most recent call last):
  File "C:/Users/xu516/PycharmProjects/Python全栈开发/第三模块/面向对象编程/32 异常处理.py", line 28, in <module>
    Foo.xxx
AttributeError: type object 'Foo' has no attribute 'xxx'

f、TypeError

for i in 3:
    pass

结果为:

Traceback (most recent call last):
  File "C:/Users/xu516/PycharmProjects/Python全栈开发/第三模块/面向对象编程/32 异常处理.py", line 30, in <module>
    for i in 3:
TypeError: 'int' object is not iterable

g、ZeroDivisionError

res1 = 1 / 0
res2 = 1 + 'str'

结果为：


Traceback (most recent call last):
  File "C:/Users/xu516/PycharmProjects/Python全栈开发/第三模块/面向对象编程/32 异常处理.py", line 33, in <module>
    res1 = 1 / 0
ZeroDivisionError: division by zero

h、KeyboardInterrupt

>>>
>>> import time
>>> time.sleep(1000)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyboardInterrupt
>>>

三、异常

强调1：错误发生条件如果是可以预知的，此时应该用if判断方法去预防异常

例如：

AGE = 10

age = input('>>: ').strip()
age = int(age)
if age > AGE:
    print('too big')

结果为:

>>: aaa
Traceback (most recent call last):
  File "C:/Users/xu516/PycharmProjects/Python全栈开发/第三模块/面向对象编程/32 异常处理.py", line 47, in <module>
    age = int(age)
ValueError: invalid literal for int() with base 10: 'aaa'

如果输入为非数字就会报错，可以用if判断避免：

AGE = 10

age = input('>>: ').strip()
if age.isdigit():
    
    age = int(age)
    if age > AGE:
        print('too big')

这样就避免抛出异常了

强调2：错误发生的条件如果是不可预知的，此时应该用异常处理机制，try......except

例如：

f = open('a.txt')

print(next(f))
print(next(f))
print(next(f))
print(next(f))
print(next(f))
print(next(f))
print(next(f))

f.close()

结果为：

111
Traceback (most recent call last):

222

  File "C:/Users/xu516/PycharmProjects/Python全栈开发/第三模块/面向对象编程/32 异常处理.py", line 60, in <module>
333

    print(next(f))
444

StopIteration
555

不知道执行到什么时候发生异常，且发生异常时后面的代码不会执行，此时使用try.......except

try:
    f = open('a.txt')

    print(next(f))
    print(next(f))
    print(next(f))
    print(next(f))
    print(next(f))
    print(next(f))
    print(next(f))

    print('after print')
    f.close()

except StopIteration:
    print('出错啦')

结果为：

111

222

333

444

555
出错啦

在try except 后面的代码还是可以正常执行的，

try:
    f = open('a.txt')

    print(next(f))
    print(next(f))
    print(next(f))
    print(next(f))
    print(next(f))
    print(next(f))
    print(next(f))

    print('after print')
    f.close()

except StopIteration:
    print('出错啦')

print('========1')
print('========2')
print('========3')

结果为：

111

222

333

444

555
出错啦
========1
========2
========3