# Collections (no typing import needed)
def process(items: list[str]) -> dict[str, int]: ...

# Optional values
def find(id: int) -> User | None: ...

# Callables
def retry(fn: Callable[[int], str], attempts: int) -> str: ...

# TypeVar for generics
type T = TypeVar('T')
def first(items: list[T]) -> T | None:
    return items[0] if items else None

# Bad: repeated validation
if not title or len(title) > 200:
    raise ValueError("Invalid title")
# ... same check elsewhere

# Good: single function
def validate_title(title: str) -> str:
    if not title or len(title) > 200:
        raise ValueError("Title must be 1-200 characters")
    return title

# Protocol for interface
class StorageBackend(Protocol):
    def save(self, key: str, data: dict) -> None: ...
    def get(self, key: str) -> dict | None: ...

# Class accepts dependency via constructor
class TaskManager:
    def __init__(self, storage: StorageBackend) -> None:
        self._storage = storage

    def create_task(self, title: str) -> Task:
        task = Task(title=title)
        self._storage.save(str(task.id), task.model_dump())
        return task

# Custom exception with context
class TaskNotFoundError(Exception):
    def __init__(self, task_id: int) -> None:
        self.task_id = task_id
        super().__init__(f"Task {task_id} not found")

# Specific catches, log errors
try:
    result = process(data)
except ValidationError as e:
    logger.warning("validation_failed", error=str(e))
    return ErrorResponse(message=str(e))
except Exception as e:
    logger.exception("unexpected_error")
    raise

src/
├── __init__.py          # Package version
├── models/              # Data classes, Pydantic models
├── services/            # Business logic
├── repositories/        # Data access
└── exceptions.py        # Custom exceptions