Function গভীরভাবে

def calculate_delivery_fee(distance_km: float, is_rainy: bool = False) -> int:
    """দূরত্ব আর আবহাওয়া দেখে delivery fee (টাকায়) বের করে।"""
    base = 30
    per_km = 10
    fee = base + int(distance_km * per_km)
    if is_rainy:
        fee += 20
    return fee


print(calculate_delivery_fee(3.5))                 # 65
print(calculate_delivery_fee(3.5, is_rainy=True))  # 85

def create_user(name: str, age: int, city: str = "Dhaka", is_admin: bool = False):
    return {"name": name, "age": age, "city": city, "is_admin": is_admin}

create_user("Rahim", 25)                          # positional
create_user(name="Rahim", age=25)                 # keyword
create_user("Rahim", 25, is_admin=True)           # মিশিয়ে, city default থাকবে
create_user(age=25, name="Rahim")                 # keyword এ ক্রম লাগে না

def transfer(amount: int, *, from_account: str, to_account: str) -> None:
    print(f"{from_account} থেকে {to_account} এ {amount} টাকা")

transfer(500, from_account="A-1", to_account="B-2")   # ঠিক আছে
# transfer(500, "A-1", "B-2")   ← TypeError, কোনটা from কোনটা to গুলিয়ে যাওয়ার সুযোগই নেই

def total(*prices: int) -> int:
    # prices একটা tuple
    return sum(prices)

print(total(100, 250, 75))     # 425

def build_query(**filters: str) -> str:
    # filters একটা dict
    parts = [f"{key}={value}" for key, value in filters.items()]
    return "&".join(parts)

print(build_query(city="Dhaka", category="food", sort="rating"))
# city=Dhaka&category=food&sort=rating

prices = [100, 250, 75]
print(total(*prices))

user_data = {"name": "Sara", "age": 28, "city": "Khulna"}
user = create_user(**user_data)    # dict এর key গুলো keyword argument হয়ে যায়

def add_item(item: str, cart: list[str] = []) -> list[str]:
    cart.append(item)
    return cart

print(add_item("চাল"))    # ['চাল']
print(add_item("ডাল"))    # ['চাল', 'ডাল']  😱 নতুন খালি cart আশা করেছিলাম

def add_item(item: str, cart: list[str] | None = None) -> list[str]:
    if cart is None:
        cart = []
    cart.append(item)
    return cart

VAT_RATE = 0.15          # global

def price_with_vat(price: int) -> float:
    total = price * (1 + VAT_RATE)   # total local, VAT_RATE global থেকে পড়া
    return total

counter = 0
def increment() -> None:
    global counter       # global variable বদলাতে চাইলে বলতে হয়
    counter += 1

def to_upper(text: str) -> str:
    return text.upper()

def shout(text: str) -> str:
    return text.upper() + "!!!"

def apply_to_all(func, items: list[str]) -> list[str]:
    return [func(item) for item in items]

names = ["rahim", "karim"]
print(apply_to_all(to_upper, names))   # ['RAHIM', 'KARIM']
print(apply_to_all(shout, names))      # ['RAHIM!!!', 'KARIM!!!']

square = lambda x: x * x     # এভাবে নাম দিয়ে রাখার বদলে def লেখাই ভালো

orders = [{"id": 1, "total": 950}, {"id": 2, "total": 300}, {"id": 3, "total": 1200}]
by_total = sorted(orders, key=lambda o: o["total"])
big = list(filter(lambda o: o["total"] > 500, orders))

def make_discount(percent: int):
    def apply(price: int) -> int:
        return price - price * percent // 100   # percent মনে রেখেছে
    return apply

eid_offer = make_discount(20)
flash_sale = make_discount(50)

print(eid_offer(1000))    # 800
print(flash_sale(1000))   # 500

# খারাপ: হিসাব আর save একসাথে, test করতে database লাগবে
def process_order(order):
    total = sum(i["price"] * i["qty"] for i in order["items"])
    if total > 1000:
        total -= 100
    db.save(order, total)

# ভালো: হিসাব আলাদা, pure
def calculate_total(items: list[dict]) -> int:
    total = sum(i["price"] * i["qty"] for i in items)
    return total - 100 if total > 1000 else total

def process_order(order, db) -> None:
    total = calculate_total(order["items"])
    db.save(order, total)