Type hints nâng cao: `Optional`, `Union`, `TypeVar`, `Generic`, `Protocol`?

from typing import Optional, Union, TypeVar, Generic, Protocol
from collections.abc import Sequence

# Optional[X] tương đương Union[X, None]
def find_user(user_id: int) -> Optional[User]:
    return db.get(user_id)  # Có thể None

# Union — nhiều types
def parse_id(value: Union[str, int]) -> int:
    return int(value)

# TypeVar — generic type variable
T = TypeVar('T')
def first(lst: Sequence[T]) -> Optional[T]:
    return lst[0] if lst else None

# Generic class
class Stack(Generic[T]):
    def __init__(self): self._items: list[T] = []
    def push(self, item: T) -> None: self._items.append(item)
    def pop(self) -> T: return self._items.pop()

# Protocol — structural subtyping (duck typing + type safety)
class Drawable(Protocol):
    def draw(self) -> None: ...

def render(obj: Drawable) -> None:
    obj.draw()  # Bất kỳ class nào có .draw() đều hợp lệ