AOP in Python With Decorators

Transcript

1. AOP in Python API design Douwe van der Meij Goldmund, Wyldebeast & Wunderliebe
2. Outline • Problem description • Aspect Oriented Programming • Implementation
3. Case • BioBench: ◦ Application for biogas installations • Enter measurements • Normalize data • Calculate plant performance
4. Problem description • Large code-base • A lot of calculations • Django web-interface • Make calculations available via an API
5. Problem description • Is there a library/framework? • Are the alternatives? • What is the actual impact of the feature? • What aspects to take into account?
6. Problem description • Aspects: ◦ Security ◦ Statistics / Logging ◦ Serialization ◦ More?
7. Problem description def add(lhs, rhs): return lhs + rhs
8. Problem description def add(lhs, rhs): return lhs + rhs def api_add(request): return add(request.args['lhs'], request.args['rhs'])
9. Problem description def add(lhs, rhs): return lhs + rhs def api_add(request): if request.args['token'] == valid: return add(request.args['lhs'], request.args['rhs']) raise Exception('Security error')
10. Problem description def add(lhs, rhs): return lhs + rhs def api_add(request): if request.args['token'] == valid: database.query().alter usage counter return add(request.args['lhs'], request.args['rhs']) raise Exception('Security error')
11. Problem description def add(lhs, rhs): return lhs + rhs def api_add(request): if request.args['token'] == valid: database.query().alter usage counter result = add(request.args['lhs'], request.args['rhs']) return '<xml>{0}</xml>'.format(result) raise Exception('Security error')
12. Problem description def add(lhs, rhs): return lhs + rhs def api_add(request): if request.args['token'] == valid: database.query().alter usage counter result = add(request.args['lhs'], request.args['rhs']) return '<xml>{0}</xml>'.format(result) raise Exception('Security error')
13. Problem description def add(lhs, rhs): return lhs + rhs def api_add(request): if request.args['token'] == valid: database.query().alter usage counter result = add(request.args['lhs'], request.args['rhs']) return '<xml>{0}</xml>'.format(result) raise Exception('Security error')
14. Problem description Code base
15. Problem description Code base • Security
16. Problem description Code base • Statistics / Logging
17. Problem description Code base • Serialization
18. Problem description Code base • Dispatcher
19. Problem description Code base • Scattered / tangled code
20. Aspect Oriented Programming • What is AOP? • Separation of concerns (aspects) • Avoid scattering / tangling
21. Code base • Scattered / tangled code Aspect Oriented Programming
22. Code base • Avoid scattering Aspect Oriented Programming
23. Code base • Avoid tangling Aspect Oriented Programming
24. Aspect Oriented Programming • How to implement these marvelous concepts? ◦ In pure python please!
25. Aspect Oriented Programming • Decorators!
26. Aspect Oriented Programming • Aspect: ◦ Pointcuts ◦ Join points ◦ Advices ▪ Before advices ▪ After advices ▪ Around advices
27. Aspect Oriented Programming • Before advice ◦ Must execute the function (no side-effects) def aspect(function): def advice(*args, **kwargs): do something here return function(*args, **kwargs) return advice
28. Aspect Oriented Programming • After advice ◦ Must execute the function (no side-effects) def aspect(function): def advice(*args, **kwargs): result = function(*args, **kwargs) do something here return result return advice
29. Aspect Oriented Programming • Around advice ◦ Allowed to bypass the function def aspect(function): def advice(*args, **kwargs): do something here result = function(*args, **kwargs) do something here return result return advice
30. Implementation • How to apply it to our case?
31. Implementation • The decorators
32. Implementation • Security aspect ◦ Around advice def secure(function): def advice(*args, **kwargs): if valid token in request object: return function(*args, **kwargs) raise Exception('No valid token provided') return advice
33. Implementation • Statistics aspect ◦ Before advice def statistics(function): def advice(*args, **kwargs): increase API usage count for the user logged in return function(*args, **kwargs) return advice
34. Implementation • Serialization aspect ◦ Around advice def serialize(function): def advice(format, *args, **kwargs): if not format in ['html', 'xml', 'json']: raise exception result = function(*args, **kwargs) make a http response of 'result' in the right format return advice
35. Implementation • Dispatcher aspect ◦ Around advice def dispatch(function): def advice(*args, **kwargs): proxy the API call to a call to the core system return advice
36. Implementation mapping = { 'api_function': (core_function, ['lhs', 'rhs']), 'create_token': (create_token, ['username', 'password']), }
37. Implementation def dispatch(function): def advice(*args, **kwargs): if API call in proxy mapping: core_function, params = mapping[API call] kwargs.update(extract(params, request)) return function(proxy=core_function, params=params, *args, **kwargs) raise exception return advice
38. Implementation • The API itself
39. Implementation @secure @serialize @statistics @dispatch def api_call(*args, **kwargs): proxy_function = kwargs['proxy'] params = kwargs['params'] return proxy_function(extract params from kwargs)
40. Conclusion • AOP offers some brilliant concepts in software engineering • Separate your concerns / aspects ◦ Avoid classical scattering and tangling
41. Questions? • Thank you! vandermeij@gw20e.com @douwevandermeij