python-design-patterns

Apply the Single Responsibility Principle by listing every reason the class could change. Split concerns from different domains, such as HTTP parsing, business rules, and data access, into separate handler, service, and repository classes with injected dependencies.

Prefer composition when building behavior from interchangeable parts, such as notification channels or formatters. Passing collaborators through the constructor makes classes flexible and easy to test with fakes, while inheritance creates rigid hierarchies that are hard to mock.

Organize code into three layers: handlers for HTTP parsing and responses, services for business logic, and repositories for data access. Dependencies must point downward only, so services never import from the API layer.

Follow the Rule of Three: wait until you have three similar instances before abstracting. Two similar-looking functions often hide different validation and error handling, so duplication is usually better than a premature or wrong abstraction.

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