from typing import Protocol

# Protocol at consumer (like Go interfaces)
class UserStore(Protocol):
    def get(self, id: str) -> User | None: ...
    def save(self, user: User) -> None: ...

class UserService:
    def __init__(self, store: UserStore):
        self.store = store  # accepts any matching impl

# GOOD: guard clauses, early returns
def process(user: User | None) -> Result:
    if user is None:
        raise ValueError("user required")
    if not user.email:
        raise ValueError("email required")
    if not is_valid_email(user.email):
        raise ValueError("invalid email")
    return do_work(user)  # happy path at end

# BAD: nested conditions
def process(user: User | None) -> Result:
    if user is not None:
        if user.email:
            if is_valid_email(user.email):
                return do_work(user)
    raise ValueError("invalid")

# Flatten complex conditionals with match
match event:
    case {"type": "click", "x": x, "y": y}:
        handle_click(x, y)
    case {"type": "key", "code": code}:
        handle_key(code)
    case _:
        raise ValueError(f"Unknown event: {event}")

# GOOD: concrete types
def process_users(users: list[User], limit: int | None = None) -> list[Result]:
    ...

# GOOD: generics when needed
from typing import TypeVar
T = TypeVar("T")
def first(items: list[T]) -> T | None:
    return items[0] if items else None

# BAD: unnecessary Any
def process(data: Any) -> Any:  # Don't do this
    ...

# except without parens (3.14)
except ValueError | TypeError:
    handle_error()

# Custom exceptions
class NotFoundError(AppError):
    def __init__(self, resource: str, id: str):
        self.resource = resource
        self.id = id
        super().__init__(f"{resource} not found: {id}")

uv sync                    # Install deps
ruff check --fix .         # Lint and autofix
ruff format .              # Format
pytest -v                  # Test
mypy .                     # Type check