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：
1 print('1') 2 print('2') 3 print('3') 4 int('aaaa') 5 print('4') 6 print('5') 7 print('6') 8 9 结果为：10 11 Traceback (most recent call last):12 113 214 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 15 316 int('aaaa')17 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：
1 print(xxx 2 3 if 1 > 3 4 5 结果为： 6 7 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 8 9 ^10 SyntaxError: unexpected EOF while parsing
2、逻辑错误
a、ValueError
1 int('aaa')2 3 结果为：4 5 Traceback (most recent call last):6 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 7 int('aaa')8 ValueError: invalid literal for int() with base 10: 'aaa' b、NameError
1 name2 3 结果为：4 5 Traceback (most recent call last):6 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, in7 name8 NameError: name 'name' is not defined
c、IndexError
1 l = [1, 2, 3]2 l[100]3 4 结果为：5 6 Traceback (most recent call last):7 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, in8 l[100]9 IndexError: list index out of range
d、KeyError
1 d = {}2 d['name']3 4 结果为：5 6 Traceback (most recent call last):7 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 8 d['name']9 KeyError: 'name' e、AttributeError
1 class Foo: 2 pass 3 4 Foo.xxx 5 6 结果为： 7 8 Traceback (most recent call last): 9 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, in10 Foo.xxx11 AttributeError: type object 'Foo' has no attribute 'xxx'
f、TypeError
1 for i in 3:2 pass3 4 结果为:5 6 Traceback (most recent call last):7 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, in8 for i in 3:9 TypeError: 'int' object is not iterable
g、ZeroDivisionError
1 res1 = 1 / 0 2 res2 = 1 + 'str' 3 4 结果为： 5 6 7 Traceback (most recent call last): 8 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, in9 res1 = 1 / 010 ZeroDivisionError: division by zero
h、KeyboardInterrupt
1 >>>2 >>> import time3 >>> time.sleep(1000)4 Traceback (most recent call last):5 File "", line 1, in 6 KeyboardInterrupt7 >>>
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：
1 AGE = 10 2 3 age = input('>>: ').strip() 4 age = int(age) 5 if age > AGE: 6 print('too big') 7 8 结果为: 9 10 >>: aaa11 Traceback (most recent call last):12 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 13 age = int(age)14 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：
1 AGE = 102 3 age = input('>>: ').strip()4 if age.isdigit():5 6 age = int(age)7 if age > AGE:8 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：
1 f = open('a.txt') 2 3 print(next(f)) 4 print(next(f)) 5 print(next(f)) 6 print(next(f)) 7 print(next(f)) 8 print(next(f)) 9 print(next(f))10 11 f.close()12 13 结果为：14 15 11116 Traceback (most recent call last):17 18 22219 20 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 21 33322 23 print(next(f))24 44425 26 StopIteration27 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
1 try: 2 f = open('a.txt') 3 4 print(next(f)) 5 print(next(f)) 6 print(next(f)) 7 print(next(f)) 8 print(next(f)) 9 print(next(f))10 print(next(f))11 12 print('after print')13 f.close()14 15 except StopIteration:16 print('出错啦')17 18 结果为：19 20 11121 22 22223 24 33325 26 44427 28 55529 出错啦 在try except 后面的代码还是可以正常执行的，
1 try: 2 f = open('a.txt') 3 4 print(next(f)) 5 print(next(f)) 6 print(next(f)) 7 print(next(f)) 8 print(next(f)) 9 print(next(f))10 print(next(f))11 12 print('after print')13 f.close()14 15 except StopIteration:16 print('出错啦')17 18 print('========1')19 print('========2')20 print('========3')21 22 结果为：23 24 11125 26 22227 28 33329 30 44431 32 55533 出错啦34 ========135 ========236 ========3