Passing default list argument to dataclasses

Question

I would like to pass default argument in my class, but somehow I am having problem:

from dataclasses import dataclass, field
from typing import List

@datac         


        

Answer 1

For complex datatypes i tend to abbreviate like so:

import copy
from dataclasses import dataclass, field
from typing import Dict, Tuple

def default_field(obj):
    return field(default_factory=lambda: copy.copy(obj))

@dataclass
class C:
    complex_attribute: Dict[str, Tuple[int, str]] = default_field({"a": (1, "x"), "b": (1, "y")})

Answer 2

From the dataclasses.field docs:

The parameters to field() are:

• default_factory: If provided, it must be a zero-argument callable that will be called when a default value is needed for this field. Among other purposes, this can be used to specify fields with mutable default values, as discussed below. It is an error to specify both default and default_factory.

Your default_factory is not a 0-argument callable but a list, which is the reason for the error:

@dataclass
class Pizza():
    ingredients: List = field(default_factory=['dow', 'tomatoes'])  # <- wrong!

Use a lambda function instead:

@dataclass
class Pizza():
    ingredients: List = field(default_factory=lambda: ['dow', 'tomatoes'])