python-design-patterns

Apply the Single Responsibility Principle by listing every reason the class could change, then split concerns from different domains into separate classes. Use constructor dependency injection so each new class receives only the collaborators it needs.

Prefer composition when behavior can be built by combining objects rather than extending classes. Composition makes dependencies explicit, simplifies testing with fakes, and avoids rigid hierarchies that break when base classes change.

Organize code into API handlers, a service layer for business logic, and a repository layer for data access. Dependencies must point downward only: handlers import services, services import repositories, never the reverse.

Follow the rule of three: wait until three similar instances exist before abstracting. Premature abstraction often produces the wrong interface, while modest duplication is cheaper to maintain than a leaky shared abstraction.

Constructors with seven or more parameters signal the class has too many responsibilities, not that injection is wrong. Split the class into smaller units first, and each resulting constructor naturally becomes smaller.