Dataclass - how to have a mutable list as field

Question

I want to use the new @dataclass decorator in my code with attribut as mutable list Something that would look like this

from dataclasses import dataclass

@dataclass
class Metrics
    accuracy: list = []
    loss: list = []

...

def do_stuff(data):
    m = Metrics()

    for i in range(0, 10):
        m.accuracy.append(accuracy_def(i))
        m.loss.append(loss_def(i))

    return m

But I have this error:

TypeError: __init__() missing 2 required positional arguments: 'accuracy' and 'loss'

Thank ! What's the != between using `list` and `List` from typing ? — Ragnar, Jan 29 '20 at 14:38
`List` is used for [type hinting](https://docs.python.org/3/library/typing.html) and allows you specify item type within a list like `List[int]` means a list of ints — hurlenko, Jan 29 '20 at 14:40

score 3 · Answer 1 · answered Jan 29 '20 at 14:33

3

Use dataclasses.field() instead

@dataclass
class Metrics:
    accuracy: List[int] = field(default_factory=list)
    loss: List[int] = field(default_factory=list)

answered Jan 29 '20 at 14:33

Filip Młynarski

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Dataclass - how to have a mutable list as field

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