Inherite dataclass using base instance Python

Question

I have two dataclasses inside bookmodel , one inherited from the other:

@dataclass(frozen=True)
class BookOrder:
    category: str
    name: str
    color: str
    price: int
    volume: int

@dataclass(frozen=True)
class ClientOrder(BookOrder):
    client_id: str

Then in another .py file, I need to init a ClientOrder instance using BookOrder :

from book_model import BookOrder
# ...

@dataclass
class Client:
    id: str
    def place_book_order(self, book_order: BookOrder):
        # want to init a ClientOrder HERE!!!

Since BookOrder is NOT callable, I cannot pass self.id to it. I was wondering if there's an elegant way to achieve this?

Update

Guess what I am trying to ask is, is there any other way to initialize a ClientOrder using BookOrder other than the below method?

client_order=ClientOrder(book_order.categry, book_order.name, ..., self.client_id)

Answer 1

One way to solve this is by not using inheritance, and instead declaring client_id as an optional attribute instead. Since the BookOrder dataclass is also frozen, we then need to call object.__setattr__ to modify the value of client_id , as mentioned in this post .

In your first module a.py :

from __future__ import annotations  # Added for compatibility with 3.7+

from dataclasses import dataclass


@dataclass(frozen=True)
class BookOrder:
    category: str
    name: str
    color: str
    price: int
    volume: int
    client_id: str | None = None

In second module b.py :

from book_model import BookOrder
# ...

@dataclass
class Client:
    id: str
    def place_book_order(self, book_order: BookOrder):
        # The following does not work, because BookOrder is a frozen
        # dataclass.
        # setattr(book_order, 'client_id', self.id)
        # Use object.__setattr__ as mentioned here:
        #  https://stackoverflow.com/a/59249252/10237506
        object.__setattr__(book_order, 'client_id', self.id)

        if book_order.client_id:
            print('Successfully set client_id attribute:',
                  repr(book_order.client_id))
        else:
            print('Was unable to set client_id attribute on frozen dataclass')


Client('abc123').place_book_order(BookOrder(*['a'] * 5))

Outputs:

Successfully set client_id attribute: 'abc123'

---

Of course, the easier way is to just not define it as frozen dataclass:

@dataclass
class BookOrder:
    category: str
    name: str
    color: str
    price: int
    volume: int
    client_id: str | None = None

And then the only change needed in b.py :

    ...

    def place_book_order(self, book_order: BookOrder):
        book_order.client_id = self.id
        ...