Architecture — Python

In addition to general architecture principles, apply these Python-specific patterns:

Code Quality

• Follow PEP-8. Use ruff for linting and formatting.
• Use type hints everywhere. Run mypy --strict in CI.
• Prefer dataclasses or pydantic for structured data over plain dicts.

Project Structure

src/
  myapp/
    domain/          # pure business logic, no I/O
      models.py
      services.py
    application/     # use cases, orchestrates domain
      use_cases.py
    infrastructure/  # DB, HTTP, filesystem
      repositories.py
      http_client.py
    presentation/    # FastAPI routes, CLI, etc.
      routes.py
  tests/
    unit/
    integration/

Patterns

• Repository: abstract data access behind a protocol/ABC.

  class UserRepository(Protocol):
      def find_by_id(self, user_id: UUID) -> User | None: ...
      def save(self, user: User) -> None: ...

• Dependency injection: accept dependencies as constructor args, not global imports.
• Factories: use @classmethod factory methods for complex initialization.
• Value objects: use @dataclass(frozen=True) for immutable domain objects.

Error Handling

• Define domain-specific exception classes in domain/exceptions.py.
• Catch broad exceptions only at the boundary (presentation layer).
• Never use bare except: — always specify the exception type.
• Log with context; re-raise when the caller needs to handle it.

Async

• Use asyncio + async/await for I/O-bound work (HTTP, DB with asyncpg/SQLAlchemy async).
• Don't mix sync and async without asyncio.run_in_executor for blocking calls.
• Use asyncio.gather for concurrent independent I/O operations.