Clean Architectures in Python - Django edition

Architectural considerations are often overlooked by developers or completely delegated to a framework. We should start once again discussing how applications are structured, how components are connected and how to lower coupling between different parts of a system, to avoid creating software that cannot easily be maintained or changed.

The “clean architecture” model predates Robert Martin, who recently brought it back to the attention of the community, and is a way of structuring applications that leverages layers separation and internal APIs to achieve a very tidy, fully-tested, and loosely coupled system. The talk introduces the main ideas of the architecture, showing how the layers can be implemented in Python, following the content of the book “Clean Architectures in Python” edited by Leanpub. The book recently reached 8,000 downloads and many readers found it useful to start learning how to test software and how to structure an application without relying entirely on the framework.

This version of the presentation includes considerations on Django and how Django and the Clean Architecture can be used together.

Transcript

1. Clean Architectures in Python A tale of durability, utility, and beauty Django edition
2. “Who wrote this code?” L E O N A R D O G I O R D A N I S O F T W A R E D E V E L O P E R A N D B L O G G E R W W W. T H E D I G I T A L C AT O N L I N E . C O M @TW_LGIORDANI - @THEDIGICAT
3. WHAT IS THE DEFINITION OF ARCHITECTURE?
4. F I R M I T A S , U T I L I T A S , V E N U S T A S Vitruvius, De architectura
5. D U R A B I L I T Y, U T I L I T Y, B E A U T Y Vitruvius, De architectura
6. T H E A R T A N D S C I E N C E I N W H I C H T H E C O M P O N E N T S O F A C O M P U T E R S Y S T E M A R E O R G A N I Z E D A N D I N T E G R AT E D
7. D O W E N E E D A R C H I T E C T U R E ?
8. Ivar Jacobson (1992) Object Oriented Software Engineering: A Use-Case Driven Approach E. Gamma, R. Helm, R. Johnson, J. Vlissides (1994) Design Patterns Robert Martin (2000) Design Principles and Design Patterns Eric Evans (2003) Domain-Driven Design: Tackling Complexity in the Heart of Software H. Hohpe, B. Woolf (2003) Enterprise Integration Patterns: Designing, Building, and Deploying Messaging Solutions
9. What is the meaning of clean?
10. You know where things are, why components are there, what something is.
11. The Clean Architecture A layered approach for a more civilized age
12. The golden rule Talk inward with simple structures, talk outwards through interfaces.
13. class Item: def __init__(self, code, price): self.code = code self.price = price Entities: simple models
14. Use case: retrieve a list of items use_case = uc.ItemsListUseCase() use_case.execute()
15. @blueprint.route('/items', methods=['GET']) def items(): pass In 2019 we are probably talking about a web application
16. @blueprint.route('/items', methods=['GET']) def items(): use_case = uc.ItemsListUseCase() use_case.execute(request.args) Incoming HTTP requests become a call and simple structures
17. @blueprint.route('/items', methods=['GET']) def items(): use_case = uc.ItemsListUseCase() use_case.execute(request.args) The use case extracts data from a repository, which can be any source of data
18. @blueprint.route('/items', methods=['GET']) def items(): use_case = uc.ItemsListUseCase() use_case.execute(request.args) And the repository can be accessed through an interface
19. @blueprint.route('/items', methods=['GET']) def items(): repo = PostgresRepo(CONNECTION_STRING) use_case = uc.ItemsListUseCase(repo) use_case.execute(request.args) The use case receives the repository interface as an argument of the call
20. class ItemsListUseCase: def __init__(self, repo): self.repo = repo def execute(self, params): # BUSINESS LOGIC HERE result = self.repo.list(params) # BUSINESS LOGIC HERE return result The use case queries the repository interface with simple structures
21. class PostgresRepo: def __init__(self, CONNECTION_STRING): self.ng = create_engine( CONNECTION_STRING) Base.metadata.bind = self.ng def list(self, filters): DBSession = sessionmaker(bind=self.ng) session = DBSession() query = ... The database interface and the database exchange data in a specific language
22. The database interface translates the specific language into simple structures and entities class PostgresRepo: def __init__(self, CONNECTION_STRING): self.ng = create_engine( CONNECTION_STRING) Base.metadata.bind = self.ng def _create_items(self, results): return [Item(code=q.code, price=q.price) for q in results] def list(self, filters): DBSession = sessionmaker(bind=self.ng) session = DBSession() query = ... return self._create_items(query.all())
23. @blueprint.route('/items', methods=['GET']) def items(): repo = PostgresRepo(CONNECTION_STRING) use_case = uc.ItemsListUseCase(repo) result = use_case.execute(request.args) The use case returns the result of the business logic: entities and simple structures
24. @blueprint.route('/items', methods=['GET']) def items(): repo = PostgresRepo(CONNECTION_STRING) use_case = uc.ItemsListUseCase(repo) result = use_case.execute(request.args) return Response( json.dumps(result), mimetype='application/json', status=200) The web framework converts entities and simple structures into HTTP responses
25. class ItemsListUseCase: def __init__(self, repo): self.repo = repo def execute(self, params): # BUSINESS LOGIC HERE result = self.repo.list(params) # BUSINESS LOGIC HERE return result Testing the use case
26. @blueprint.route('/items', methods=['GET']) def items(): repo = PostgresRepo(CONNECTION_STRING) use_case = uc.ItemsListUseCase(repo) result = use_case.execute(request.args) return Response( json.dumps(result), mimetype='application/json', status=200) Testing the HTTP endpoint
27. Testing the repository interface: integration test class PostgresRepo: def __init__(self, CONNECTION_STRING): self.ng = create_engine( CONNECTION_STRING) Base.metadata.bind = self.ng def _create_items(self, results): return [Item(code=q.code, price=q.price) for q in results] def list(self, filters): DBSession = sessionmaker(bind=self.ng) session = DBSession() query = ... return self._create_items(query.all())
28. HOW CAN WE USE THE CLEAN ARCHITECTURE WITH DJANGO?
29. The Clean Architecture The Django Architecture
30. The Clean Architecture The Django Architecture
31. The Clean Architecture The Django Architecture
32. The Clean Architecture The Django Architecture
33. The Clean Architecture The Django Architecture
35. IT DEPENDS
36. LOAD BALANCERS TO THE RESCUE
37. Is it possible to migrate an existing system?
38. Is this the definitive architecture?
39. Clean Architectures in Python A practical approach to better software design bit.ly/getpycabook Leonardo Giordani
40. Harry Percival, Bob Gregory github.com/python-leap/book Pythonic Application Architecture Patterns (working title)
41. Thank you! @tw_lgiordani - @thedigicat - bit.ly/getpycabook https://speakerdeck.com/lgiordani