Python Type Safety

def get_user(user_id: str) -> User | None:    """Return type makes 'might not exist' explicit."""    ...
# Type checker enforces handling None caseuser = get_user("123")if user is None:    raise UserNotFoundError("123")print(user.name)  # Type checker knows user is User here

def get_user(user_id: str) -> User:    """Retrieve user by ID."""    ...
def process_batch(    items: list[Item],    max_workers: int = 4,) -> BatchResult[ProcessedItem]:    """Process items concurrently."""    ...
class UserRepository:    def __init__(self, db: Database) -> None:        self._db = db
    async def find_by_id(self, user_id: str) -> User | None:        """Return User if found, None otherwise."""        ...
    async def find_by_email(self, email: str) -> User | None:        ...
    async def save(self, user: User) -> User:        """Save and return user with generated ID."""        ...

# Preferred (3.10+)def find_user(user_id: str) -> User | None:    ...
def parse_value(v: str) -> int | float | str:    ...
# Older style (still valid, needed for 3.9)from typing import Optional, Union
def find_user(user_id: str) -> Optional[User]:    ...

def process_user(user_id: str) -> UserData:    user = find_user(user_id)
    if user is None:        raise UserNotFoundError(f"User {user_id} not found")
    # Type checker knows user is User here, not User | None    return UserData(        name=user.name,        email=user.email,    )
def process_items(items: list[Item | None]) -> list[ProcessedItem]:    # Filter and narrow types    valid_items = [item for item in items if item is not None]    # valid_items is now list[Item]    return [process(item) for item in valid_items]

from typing import TypeVar, Generic
T = TypeVar("T")E = TypeVar("E", bound=Exception)
class Result(Generic[T, E]):    """Represents either a success value or an error."""
    def __init__(        self,        value: T | None = None,        error: E | None = None,    ) -> None:        if (value is None) == (error is None):            raise ValueError("Exactly one of value or error must be set")        self._value = value        self._error = error
    @property    def is_success(self) -> bool:        return self._error is None
    @property    def is_failure(self) -> bool:        return self._error is not None
    def unwrap(self) -> T:        """Get value or raise the error."""        if self._error is not None:            raise self._error        return self._value  # type: ignore[return-value]
    def unwrap_or(self, default: T) -> T:        """Get value or return default."""        if self._error is not None:            return default        return self._value  # type: ignore[return-value]
# Usage preserves typesdef parse_config(path: str) -> Result[Config, ConfigError]:    try:        return Result(value=Config.from_file(path))    except ConfigError as e:        return Result(error=e)
result = parse_config("config.yaml")if result.is_success:    config = result.unwrap()  # Type: Config