from pydantic import BaseModel, Field

# Base Schema
class BookBase(BaseModel):
    title: str = Field(..., description="Title of the book")
    author: str = Field(..., description="Author of the book")
    description: str = Field(..., description="Description of the book")
    published_year: int = Field(..., gt=0, description="Year the book was published")

# Schema for creating a new Book
class BookCreate(BookBase):
    isbn: str = Field(..., description="ISBN of the book")

    class Config:
        schema_extra = {
            "example": {
                "title": "The Great Gatsby",
                "author": "F. Scott Fitzgerald",
                "description": "A novel about the decadence of the Jazz Age.",
                "published_year": 1925,
                "isbn": "978-0743273565"
            }
        }

# Schema for Book responses
class Book(BookBase):
    id: int
    isbn: str

    class Config:
        orm_mode = True
```

This code defines three Pydantic schemas for the Book entity:

1. `BookBase`: A base schema containing common fields for Book.
2. `BookCreate`: A schema for creating a new Book, inheriting from `BookBase` and adding the `isbn` field.
3. `Book`: A schema for Book responses, inheriting from `BookBase` and adding the `id` and `isbn` fields.

The `Field` class is used to add validation and descriptions to the fields. For example, `published_year` is validated to be greater than 0, and `isbn` is required for creating a new Book.

The `Config` class is used to provide examples for the `BookCreate` schema and to enable the `orm_mode` for the `Book` schema, which is useful when working with ORMs like SQLAlchemy.