Use type hints in all function signatures, including return types. Prefer concrete types over Any; reach for Protocol when you really need duck typing. Run mypy or pyright in CI.

Dataclasses and Pydantic

Use dataclasses for simple data containers; reach for Pydantic when you need runtime validation at a system boundary. Do not use Pydantic internally where dataclasses suffice — the validation cost is real.

Imports

Sort imports with isort/ruff. One module per import line. Group: stdlib, third-party, first-party, separated by blank lines.

SQL safety

Always use parameterized queries for every database access. Never concatenate user-provided strings into SQL statements directly. This includes dynamic WHERE clauses, ORDER BY clauses, and table names.

Logging

Never log secrets, personally identifiable information, or full request or response bodies. Scrub sensitive fields at the logging boundary using a structured logger.

Boundary discipline

Treat any data that crossed a network or process boundary as untrusted until proven otherwise. Apply the same scrutiny to data from other internal services as you would to data from the public internet. The threat model assumes lateral movement; act accordingly across every service-to-service hop in the system.