Question

在Python 3.7中，我可以使用默认的InitVar创建一个 dataclass ：

from dataclasses import dataclass, InitVar, field

@dataclass
class Foo:
    seed: InitVar[str] = field(default='tomato')
    stored: str = field(init=False)

    def __post_init__(self, seed: str):
        self.stored = f'planted {seed}'

print(Foo())

现在，我尝试创建一个具有可变默认值的类似 dataclass ，为此，我需要使用default_factory来代替：

from dataclasses import dataclass, InitVar, field
from typing import List

@dataclass
class Bar:
    seeds: InitVar[List[str]] = field(default_factory=list)
    stored: List[str] = field(init=False)

    def __post_init__(self, seeds: List[str]):
        self.stored = [f'planted {seed}' for seed in seeds]

print(Bar())

但是，这是无效的。 Python引发TypeError: field seeds cannot have a default factory。

标准库中的dataclasses.py文件无法解释原因：

    # Special restrictions for ClassVar and InitVar.
    if f._field_type in (_FIELD_CLASSVAR, _FIELD_INITVAR):
        if f.default_factory is not MISSING:
            raise TypeError(f'field {f.name} cannot have a '
                            'default factory')
        # Should I check for other field settings? default_factory
        # seems the most serious to check for.  Maybe add others.  For
        # example, how about init=False (or really,
        # init=<not-the-default-init-value>)?  It makes no sense for
        # ClassVar and InitVar to specify init=<anything>.

为什么？这项特殊限制的依据是什么？这有什么意义？

在Python数据类中，为什么InitVar可以具有默认值但没有default_factory？

0 个答案: