将默认列表参数传递给数据类

Question

我想在我的班级中传递默认参数,但不知何故我遇到了问题:

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)

如果我打字

>>> 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

我收到以下错误:

-------------------------------------------------- ------------------------- AttributeError Traceback(最近一次调用last)in()----> 1 Pizza.ingredients

AttributeError:类型对象'Pizza'没有属性'ingredients'

编辑:对于类实例我收到以下错误:

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)

-------------------------------------------------- ------------------------- TypeError Traceback(最近一次调用last)in()----> 1 my_order = Pizza()

在init(自我,成分,肉)

TypeError:'list'对象不可调用

编辑2:

快速检查Pizza以下产量:

>>> 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

有Pizza但不Pizza

Answer 1

从该dataclasses.field文档:

参数field()是:

• default_factory:如果提供,则必须是零参数可调用,当此字段需要默认值时,将调用该调用.除了其他目的之外,这可以用于指定具有可变默认值的字段,如下所述.指定default和default_factory都是错误的.

你default_factory不是一个0参数可调用但是一个列表,这是错误的原因:

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

使用lambda函数代替:

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

Answer 2

对于复杂的数据类型，我倾向于这样缩写：

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")})