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

@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?

Answer 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:

@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'])