Конфигурация, переменные окружения и архитектура приложения

import os

os.environ["DATABASE_URL"] = "postgresql://shop:secret@db:5432/shop"   # обычно задаёт платформа, а не код
os.environ["DEBUG"] = "1"

database_url = os.environ["DATABASE_URL"]            # обязательная: KeyError, если нет — упадём сразу при старте
debug = os.getenv("DEBUG", "0") == "1"               # необязательная с умолчанием
workers = int(os.getenv("WORKERS", "4"))             # всё в окружении — строки, приводим сами
print(database_url, debug, workers)
print(bool("0"), bool("false"))                      # ловушка: непустая строка — всегда True

from pydantic import BaseModel, Field, SecretStr, ValidationError, field_validator

class Settings(BaseModel):
    database_url: str
    secret_key: SecretStr
    debug: bool = False
    allowed_hosts: list[str] = []
    workers: int = Field(4, ge=1, le=64)

    @field_validator("allowed_hosts", mode="before")
    @classmethod
    def split_csv(cls, v):
        return [h.strip() for h in v.split(",") if h.strip()] if isinstance(v, str) else v

env = {"DATABASE_URL": "postgresql://db/shop", "SECRET_KEY": "s3cr3t", "DEBUG": "yes", "ALLOWED_HOSTS": "a.ru, b.ru"}
settings = Settings(**{k.lower(): v for k, v in env.items()})
print(settings)
print(settings.secret_key.get_secret_value())

try:
    Settings(database_url="x", secret_key="k", debug="может быть", workers="100")
except ValidationError as e:
    print([f"{err['loc'][0]}: {err['msg']}" for err in e.errors()])

from dataclasses import dataclass
from typing import Protocol

# --- домен ---
class DomainError(Exception):
    code = "domain_error"

class NotEnoughStock(DomainError):
    code = "not_enough_stock"

@dataclass
class Product:
    id: int
    title: str
    price: int
    stock: int

# --- порт: что сервису нужно от хранилища ---
class ProductRepository(Protocol):
    def get(self, product_id: int) -> Product | None: ...
    def save(self, product: Product) -> None: ...

# --- сервис: бизнес-логика, не знает ни про HTTP, ни про SQL ---
class OrderService:
    def __init__(self, products: ProductRepository, discount_percent: int = 0):
        self.products = products
        self.discount_percent = discount_percent

    def place_order(self, product_id: int, quantity: int) -> int:
        product = self.products.get(product_id)
        if product is None:
            raise LookupError(f"товар {product_id} не найден")
        if product.stock < quantity:
            raise NotEnoughStock(f"осталось {product.stock} шт.")
        product.stock -= quantity
        self.products.save(product)
        return product.price * quantity * (100 - self.discount_percent) // 100

# --- адаптер: реализация для тестов (в проде — на SQLAlchemy) ---
class InMemoryProducts:
    def __init__(self, *items: Product):
        self.items = {p.id: p for p in items}
    def get(self, product_id):
        return self.items.get(product_id)
    def save(self, product):
        self.items[product.id] = product

# --- «API»: переводит исключения домена в HTTP-коды ---
def handle_post_order(service: OrderService, payload: dict) -> tuple[int, dict]:
    try:
        total = service.place_order(payload["product_id"], payload["quantity"])
    except NotEnoughStock as e:
        return 409, {"error": e.code, "message": str(e)}
    except LookupError as e:
        return 404, {"error": "not_found", "message": str(e)}
    return 201, {"total": total}

repo = InMemoryProducts(Product(1, "Кружка", 500, stock=3))
service = OrderService(repo, discount_percent=10)          # внедрение зависимостей через конструктор
print(handle_post_order(service, {"product_id": 1, "quantity": 2}))
print(handle_post_order(service, {"product_id": 1, "quantity": 2}))
print(handle_post_order(service, {"product_id": 7, "quantity": 1}))