first option to mutate

main.py

import dataclasses
from dataclasses import dataclass
from decimal import Decimal


@dataclass(frozen=True)
class Account:
    amount: Decimal
    blocked: Decimal

    def add_amount(self, amount: Decimal) -> "Account":
        return Account(
            amount=self.amount + amount,
            blocked=self.blocked,
        )

    def block_amount(self, amount: Decimal) -> "Account":
        if amount > self.amount:
            raise ValueError("Insufficient balance")
        return Account(
            amount=self.amount,
            blocked=self.blocked + amount,
        )

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