Python异常处理（上）

一.教程

1. #常规异常处理
2. def spam(divideBy):
3. try:
4. return 42 / divideBy
5. except ZeroDivisionError:
6. print('Error: Invalid argument.')
7. try:
8. print(5/0)
9. except ZeroDivisionError:
10. print("you can't divide by zero!")
11. else:
12. print("ok")
13. try: #文件不存在
14. with open(filename) as lin_file:
15.         a = lin_file.read()
16. except FileNotFoundError:
17. print("file not found")
18. raise Exception('This is the error message.') #返回错误
19. #保存反向追踪
20. import traceback
21. try:
22. #raise Exception('This is the error message.')
23. print(abc) #一个不存在变量
24. except:
25.     errorFile = open('errorInfo.txt', 'w')
26.     errorFile.write(traceback.format_exc())
27.     errorFile.close()
28. print('The traceback info was written to errorInf')
29. #异常
30. except Exception as other: #将异常赋值给变量
31. class UppercaseException(Exception):
32. pass
33. words = ['eeenie', 'meenie', 'miny', 'Mo']
34. for word in words:
35. if word.isupper():
36. raise UppercaseException(word)
37. for place in sys.path:
38. print(place)

二.断言

1. market_2nd = {'ns': 'green', 'ew': 'red', 'xx': 'yellow'}
2. mission_16th = {'ns': 'red', 'ew': 'green'}
3. def switchLights(stoplight):
4. for key in stoplight.keys():
5. if stoplight[key] == 'green':
6.             stoplight[key] = 'yellow'
7. elif stoplight[key] == 'yellow':
8.             stoplight[key] = 'red'
9. elif stoplight[key] == 'red':
10.             stoplight[key] = 'green'
11. assert 'red' in stoplight.values(), 'Neither light is red! ' + str(stoplight)
12. switchLights(market_2nd)
13. switchLights(mission_16th) #ns会变为绿的，ew方向为成为黄色，导致没有红色的，车会一直开，会拥堵
14. #执行时-O则禁用断言，不过通常是注释断言