python-3.x - 如何获取python3数据类实例的索引？

Question

我想限制数据类实例的最大数量并了解实例的索引。这是我想要的行为:

Veget('tomato', 2.4, 5)
Veget('salad', 3.5, 2)
Veget('carot', 1.2, 7)

for Veget in Veget.instances:
    print(Veget)

Veget(index=0, name='tomato', price=2.4, quantity=5)
Veget(index=1, name='salad', price=3.5, quantity=2)
Veget(index=2, name='carot', price=1.2, quantity=7)

我尝试了以下方法，它确实处理了创建限制:

from dataclasses import dataclass

MAX_COUNT = 3


class Limited:

    instances = []

    def __new__(cls, *_):
        if len(cls.instances) < MAX_COUNT:
            newobj = super().__new__(cls)
            cls.instances.append(newobj)
            return newobj
        else:
            raise RuntimeError('Too many instances')

@dataclass
class Veget(Limited):
    name: str
    price: float
    quantity: int

但打印时不会显示索引:

Veget(name='tomato', price=2.4, quantity=5)
Veget(name='salad', price=3.5, quantity=2)
Veget(name='carot', price=1.2, quantity=7)

Answer 1

对数据类进行隐式限制或要求某种验证通常是通过指定的__post_init__来实现的。而不是使用对象继承。利用它的实现可能如下所示，在我看来，这更容易维护和理解:

from dataclasses import dataclass, field

MAX_COUNT = 3
VEGET_INDEX = []


@dataclass
class Veget:
    index: int = field(init=False)
    name: str
    price: float
    quantity: int

    def __post_init__(self):
        self.index = len(VEGET_INDEX)
        if self.index >= MAX_COUNT:
            raise RuntimeError("Too many instances")
        VEGET_INDEX.append(self)

您还可以使用计数器代替在初始化后例程中递增的列表，但引用列表似乎对于调试目的很方便。无论如何，创建三个允许的实例并尝试创建第四个实例将如下所示:

>>> Veget('tomato', 2.4, 5)
Veget(index=0, name='tomato', price=2.4, quantity=5)
>>> Veget('salad', 3.5, 2)
Veget(index=1, name='salad', price=3.5, quantity=2)
>>> Veget('carot', 1.2, 7)
Veget(index=2, name='carot', price=1.2, quantity=7)
>>> Veget('potato', 0.7, 3)
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "<string>", line 5, in __init__
  File "<input>", line 17, in __post_init__
RuntimeError: Too many instances