我想限制数据类实例的最大数量,并知道实例的索引。在这里,我不知道如何获得这种行为:
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)
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
索引属性未出现,我知道了
Veget(name='tomato', price=2.4, quantity=5)
Veget(name='salad', price=3.5, quantity=2)
Veget(name='carot', price=1.2, quantity=7)
对数据类施加隐式限制或要求某种验证通常是通过指定的__post_init__
而不是使用对象继承来实现的。这样的实现看起来像这样,在我看来,这将更易于维护和理解:
__post_init__
您还可以使用计数器来代替在post init例程中递增的列表,但是对于调试目的,引用列表似乎很方便。无论如何,创建三个允许的实例并尝试创建第四个实例如下所示:
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)