add basic entity layer

src/entities/cart.py

@@ -0,0 +1,27 @@
+#python imports
+from typing import List
+from dataclasses import dataclass
+
+#dependency imports
+from entities.cart_item import CartItem
+from entities.product import Product
+
+@dataclass
+class Cart:
+    items: List[CartItem]
+
+    def add_item(self, product: Product, quantity: int):
+        for item in self.items:
+            if item.product.id == product.id:
+                item.quantity += quantity
+                return
+        self.items.append(CartItem(product=product, quantity=quantity))
+
+    def remove_item(self, product_id: str):
+        self.items = [item for item in self.items if item.product.id != product_id]
+
+    def calculate_total_price(self) -> float:
+        return sum(item.calculate_total_price() for item in self.items)
+
+    def list_items(self) -> List[str]:
+        return [f"{item.quantity} x {item.product.name} - {item.calculate_total_price()} EUR" for item in self.items]

src/entities/cart_item.py

@@ -0,0 +1,17 @@
+#python imports
+from typing import List
+from dataclasses import dataclass
+
+@dataclass
+class CartItem:
+    product_id: int
+    name: str
+    quantity: int
+    price: float
+
+    def post_init(self):
+        if self.quantity <= 0:
+            raise ValueError("Quantity has to be atleast 1")
+
+    def calculate_total_price(self) -> float:
+        return self.quantity * self.price

src/entities/product.py

@@ -0,0 +1,13 @@
+#python imports
+from dataclasses import dataclass
+
+@dataclass
+class Product:
+    id: str
+    name: str
+    description: str
+    price: float
+
+    def post_init(self):
+        if self.price < 0:
+            raise ValueError("Der Preis darf nicht negativ sein.")