architecting-python

Functional core and imperative shell is an architectural pattern that separates pure business logic from side effects, allowing Python domain logic to remain highly testable while the outer shell coordinates IO operations and enforces boundaries.

Structure Python projects using hexagonal architecture by organizing code into ports and adapters, enabling plug-and-play infrastructure components that connect to the central domain through explicit boundaries and Pydantic request-response models.

Use Pydantic for domain modeling in Python by defining immutable Value Objects and clear Request/Response models to validate inputs, translate external data into domain concepts, and enforce type safety across architectural boundaries.

Separate business logic from IO in Python applications when you need scalable, maintainable codebases with clear boundaries between domain and infrastructure, ensuring pure functions remain testable without external dependencies.

Python functional core architecture works for large projects by applying hexagonal architecture guidance and explicit typing, enabling teams to design scalable codebases with plug-and-play infrastructure and disciplined module structuring.

Limitations of functional core in Python include the discipline required to maintain pure functions without side effects and the overhead of mapping external IO through imperative shell boundaries and Pydantic models for every domain interaction.