【python】程序常见异常&异常处理方法

常见的异常

1. 除0运算 - ZeroDivisionError
   1/0
   division by zero

2. 找不到文件 - FileNotFoundError
   with open(“tmp.txt”) as f:
   pass
   No such file or directory: ‘tmp.txt’

3. 值错误 - ValueError
   传入一个调用者不期望的值，即使这个值的类型是正确的
   s = “1.6”
   n = int(s)
   invalid literal for int() with base 10: ‘1.6’

4. 索引错误 - IndexError
   ls = [0,1,2,3]
   ls[5]
   list index out of range

5. 类型错误 - TypeError
   传入对象类型与要求不符
   2 + “3”
   unsupported operand type(s) for +: ‘int’ and ‘str’

6. 名称错误 - NameError
   使用一个未被定义的变量
   print(a)
   NameError : name ‘a’ is not defined

7. 键错误 - KeyError
   试图访问字典里不存在的键
   d = {}
   d[“m”]
   KeyError: ‘m’

当异常发生的时候，如果不预先设定处理方法，程序就会中断

异常的处理

目的：提高程序的稳定性和可靠性

try_except

1. 单分支

   x, y = 5, 0
   try:
   	z = x / y
   except ZeroDivisionError:
   	print('0不可以被除')
   1
   2
   3
   4
   5

2. 多分支

   d = {'name': 'Tom'}
   try:
   	d['height']
   except NameError:
   	print('变量名不存在')
   except KeyError:
   	print('键不存在')
   1
   2
   3
   4
   5
   6
   7

3. 万能异常Exception

   d = {'name': 'Tom'}
   try:
   	d['height']
   except Exception:
   	print('出错了')
   1
   2
   3
   4
   5

4. 捕获异常的值as

   d = {'name': 'Tom'}
   try:
   	d['height']
   except Exception as e:
   	print(e)
   1
   2
   3
   4
   5

try_except_else

try顺利执行，则else也执行

try:
	with open('tmp.csv') as f:
		text = f.read()
except FileNotFoundError:
	print('找不到该文件')
else:
	print('找到了该文件')
1
2
3
4
5
6
7

try_except_finally

无论try执行与否，finally都执行

d = {'name': 'Tom'}
try:
	d['height']
except Exception as e:
	print(e)
finally:
	print('无论是否出发异常，都将执行')
1
2
3
4
5
6
7