Optional[Type] = Union[Type, None]

from typing import Optional

def find_user(user_id: int) -> Optional[str]:
    user_db = {1: "Alice", 2: "Bob"}
    return user_db.get(user_id)  # 找不到時(shí)返回None

# 創(chuàng)建test.py
def get_phone(user: Optional[dict]) -> Optional[str]:
    if user is None:
        return None
    return user.get("phone")  # 這里安全訪(fǎng)問(wèn)！

# 運(yùn)行類(lèi)型檢查：mypy test.py

from typing import Union

# 以下兩種聲明等價(jià)：
def func1() -> Optional[int]: ...
def func2() -> Union[int, None]: ...

import requests
from typing import Optional, Dict

def fetch_user_data(url: str) -> Optional[Dict]:
    try:
        response = requests.get(url, timeout=3)
        return response.json() if response.status_code == 200 else None
    except requests.exceptions.RequestException:
        return None

def process_data():
    data = fetch_user_data("https://api.example.com/users/42")
    if data is None:
        print("數(shù)據(jù)獲取失敗，啟動(dòng)備用方案")
        return
    
    # 安全操作：此時(shí)data一定是dict類(lèi)型
    print(f"用戶(hù)名: {data.get('name', '未知')}")

class Wallet:
    def __init__(self, balance: Optional[float] = None):
        self.balance = balance

class User:
    def __init__(self, wallet: Optional[Wallet] = None):
        self.wallet = wallet

def get_balance(user: Optional[User]) -> Optional[float]:
    return user.wallet.balance if user and user.wallet else None

# 測(cè)試鏈?zhǔn)秸{(diào)用
user1 = User(Wallet(100.0))
user2 = User()  # 沒(méi)有錢(qián)包
user3 = None    # 無(wú)用戶(hù)對(duì)象

print(get_balance(user1))  # 100.0
print(get_balance(user2))  # None
print(get_balance(user3))  # None

# 源碼真相（typing.py）：
Optional = Union[T, None]

# 編譯后類(lèi)型擦除
import dis
def demo(x: Optional[int]) -> Optional[str]:
    return str(x) if x is not None else None

dis.dis(demo)
"""
  2           0 LOAD_FAST                0 (x)
              2 LOAD_CONST               0 (None)
              4 IS_OP                    0   # 關(guān)鍵比較操作
              6 POP_JUMP_IF_TRUE        12
              8 LOAD_GLOBAL              0 (str)
             10 LOAD_FAST                0 (x)
             12 CALL_FUNCTION            1
             14 RETURN_VALUE
        >>   16 LOAD_CONST               0 (None)
             18 RETURN_VALUE
"""

# 危險(xiǎn)代碼！
def print_name(user: Optional[User]):
    print(user.name)  # 如果user=None，直接崩潰！

# 正確姿勢(shì)
def print_name_safe(user: Optional[User]):
    if user is None:
        print("匿名用戶(hù)")
        return
    print(user.name)

def fetch_data() -> Optional[Optional[str]]:
    return None  # 或返回"Hello" 或返回None

result = fetch_data()
# 需要兩層判斷！
if result is not None:
    if result is not None:  # 反模式！
        ...

def safe_divide(a: float, b: Optional[float]) -> Optional[float]:
    # 1. 顯式檢查None
    if b is None or b == 0:
        return None
    # 2. 使用類(lèi)型守衛(wèi)
    assert isinstance(b, float), "b必須是浮點(diǎn)數(shù)"
    # 3. 返回合理默認(rèn)值
    return a / b

from dataclasses import dataclass
from typing import Optional

@dataclass
class Product:
    id: int
    name: str
    price: Optional[float] = None  # 明確標(biāo)注可選字段

book = Product(id=1, name="Python圣經(jīng)")
if book.price is None:
    print("價(jià)格待定")

from typing import cast, Optional

def handle_data(data: object) -> Optional[int]:
    if isinstance(data, int):
        return cast(Optional[int], data)  # 顯式類(lèi)型轉(zhuǎn)換
    return None

result: Optional[int] = get_value()

# 方案1：顯式檢查
if result is not None:
    ...

# 方案2：提供默認(rèn)值
value = result if result is not None else 0

# 方案3：使用Walrus運(yùn)算符（Python 3.8+）
if (result := get_value()) is not None:
    ...

def new_optional(user: str | None) -> int | None: ...