A mutable dataclass

main.py

from dataclasses import dataclass
from decimal import Decimal


@dataclass
class Account:
    amount: Decimal
    blocked: Decimal

    def add_amount(self, amount: Decimal) -> None:
        self.amount += amount

    def block_amount(self, amount: Decimal) -> None:
        if amount > self.amount:
            raise ValueError("Insufficient balance")
        self.blocked += amount

    @property
    def available_amount(self) -> Decimal:
        return self.amount - self.blocked