Clean Architectures in Python

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.

Transcript

1. Clean Architectures in Python A tale of durability, utility, and beauty
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 S 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. Clean Architectures in Python A practical approach to better software design bit.ly/getpycabook Leonardo Giordani
12. The Clean Architecture A layered approach for a more civilized age
13. The golden rule Talk inward with simple structures, talk outwards through interfaces.
14. class Item: def __init__(self, code, price): self.code = code self.price = price Entities: simple models
15. Use case: retrieve a list of items use_case = uc.ItemsListUseCase() use_case.execute()
16. @blueprint.route('/items', methods=['GET']) def items(): pass We want to build a web application
17. @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
18. @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
19. @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
20. @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
21. 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
22. 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
23. 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())
24. @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
25. @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
26. 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
27. @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
28. 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())
29. Is it possible to migrate an existing system?
30. Is this the definitive architecture?
31. Clean Architectures in Python A practical approach to better software design bit.ly/getpycabook Leonardo Giordani
32. Harry Percival, Bob Gregory www.cosmicpython.com Architecture Patterns with Python
33. Thank you! @tw_lgiordani - @thedigicat bit.ly/getpycabook - thedigitalcatonline.com