How to mutate dataclass argument upon instantiation?

Question

I have a dataclass that looks like this:

@dataclass
class AllLastTick:
    tick_type_mapping = {
        0: "bid_size",
        1: "bid_price",
        2: "ask_price",
        3: "ask_size",
        4: "last_trade_price",
        5: "last_trade_size"
    }
    time: int
    tick_type: int
    price: float
    size: Decimal

And I instantiate it like this:

tick_data = (
    time,
    tick_type,
    price,
    size
)

tick = AllLastTick(*tick_data)

The result is something that looks like this:

AllLastTick(time=1699358716, tick_type=2, price=178.49, size=Decimal('200'))

What I'd like to do is convert the tick_type argument to the value in AllLastTick .tick_type_mapping when the class is instantiated.

The ideal result would look like this:

AllLastTick(time=1699358716, tick_type="ask_price", price=178.49, size=Decimal('200'))

I've tried using something like this:

...
tick_type: int = field(default_factory=lambda x: tick_type_mapping[x])
...

Which I understand does not work because the fields cannot be mutable (question mark on this point).

How can I accomplish this?

Answer 1

How about using __post_init__? See the docs.

from dataclasses import dataclass, field
from decimal import Decimal

class AllLastTick:
    tick_type_mapping = {
        0: "bid_size",
        1: "bid_price",
        2: "ask_price",
        3: "ask_size",
        4: "last_trade_price",
        5: "last_trade_size"
    }
    time: int
    tick_idx: int
    tick_type: str = field(init=False)
    price: float
    size: Decimal
    def __post_init__(self):
        self.tick_type = self.tick_type_mapping[self.tick_idx]

Then,

tick = AllLastTick(*tick_data)

Will use the index and define a tick type for you. You just need to store the key in another attribute (tick_idx).