Passing default list argument to dataclasses

Solution 1:

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:

from dataclasses import dataclass, field
from typing import List

@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'])

Solution 2:

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")})