python-design-patterns

Apply the Single Responsibility Principle by listing every change that could require editing the class. If changes come from different domains like HTTP parsing and business rules, split the class into focused units such as a handler, service, and repository.

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

Organize code into an API layer for request parsing, a service layer for business logic, and a repository layer for data access. Dependencies should only point downward, so services never import from handlers.

Follow the Rule of Three: wait until a pattern appears three times before abstracting, since premature abstraction often creates the wrong interface. However, if duplicated copies are already diverging and causing bugs, extract the shared behavior immediately.

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