python - Passing default list argument to dataclasses

35
votes

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

@dataclass
class Pizza():
    ingredients: List = field(default_factory=['dow', 'tomatoes'])
    meat: str = field(default='chicken')

    def __repr__(self):
        return 'preparing_following_pizza {} {}'.format(self.ingredients, self.meat)

If I now try to instantiate Pizza, I get the following error:

>>> my_order = Pizza()
Traceback (most recent call last):
  File "pizza.py", line 13, in <module>
    Pizza()
  File "<string>", line 2, in __init__
TypeError: 'list' object is not callable

What am I doing wrong?

2
pythonpython-3.xooptype-hinting
I think the problem may be because you haven't created an instance of your class - N Chauhan
even with created class instance it does not work.. - H.Bukhari
No repro. It works perfectly fine with an instance. Please post a minimal reproducible example so we can see what you're doing wrong. - Aran-Fey
@Aran-Fey did you try to write the code? As I said it doesn't work with the an instance, see my edit - H.Bukhari
I dont want to use init, the whole point of @dataclass decorator is that you can skip init - H.Bukhari

2 Answers

66
votes

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'])
10
votes

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")})