feat(book): create book class

src/bookstore/book.py

@@ -0,0 +1,22 @@
+from attrs import define, field, setters, validators
+
+from .isbn import ISBN
+
+
+def _check_price_value(instance, attribute, value):
+    if value < 0:
+        raise ValueError("Price must be larger or equal to 0!")
+
+
+def _check_stock_value(instance, attribute, value):
+    if value < 0:
+        raise ValueError("Stock must be larger or equal to 0!")
+
+
+@define
+class Book:
+    title: str = field()
+    author: str = field()  # TODO: add default author as "Unknown"
+    isbn: ISBN = field(on_setattr=setters.frozen, repr=lambda value: f"'{value}'")
+    price: float = field(converter=float, validator=[validators.instance_of(float), _check_price_value])
+    stock: int = field(converter=int, validator=[validators.instance_of(int), _check_stock_value])