英文:
how can I narrow dataclass annotations (i.e., how can I update type hints after handling default None in post_init)?
问题
以下是代码的中文翻译部分：
我有一个数据类，它可以接受一个关键字值，或者如果未指定值，则可以从其他属性推断出一个值。
import dataclasses
@dataclasses.dataclass
class RelatedValues:
primary: float
_: dataclasses.KW_ONLY
secondary: float | None = None
def __post_init__(self):
if self.secondary is None:
self.secondary = self.primary
这段代码有效，但它让我在`.secondary`的类型提示中被困扰，即使在`__post_init__`之后`.secondary`*绝对不可能*是`None`。
在`__post_init__`中对`self.secondary`进行`cast`操作不起作用。但这个方法起作用：
NULL_FLOAT = float(int(uuid.uuid4())
@dataclasses.dataclass
class RelatedValues:
primary: float
_: dataclasses.KW_ONLY
secondary: float = NULL_FLOAT
def __post_init__(self):
if self.secondary == NULL_FLOAT:
self.secondary = self.primary
但这种方法感觉不太符合Python的风格。
以下方法也有效：
@dataclasses.dataclass
class RelatedValues:
primary: float
_: dataclasses.KW_ONLY
_secondary: float | None = None
def __post_init__(self):
if self._secondary is None:
self.secondary = self.primary
else:
self.secondary = self._secondary
或者这种方法也可以：
@dataclasses.dataclass
class RelatedValues:
primary: float
_: dataclasses.KW_ONLY
_secondary: float | None = None
@property
def secondary(self) -> float:
if self._secondary is None:
self.secondary = self.primary
else:
self.secondary = self._secondary
但后两种方法只是为了类型缩小而操纵我的kwargs，这种做法有点不太对。
我漏掉了什么？
英文:
I have a dataclass that can take a keyword value, or, if no value is specified, infer a value from other attributes.
import dataclasses
@dataclasses.dataclass
class RelatedValues:
primary: float
_: dataclasses.KW_ONLY
secondary: float | None = None
def __post_init__(self):
if self.secondary is None:
self.secondary = self.primary
This code works, but it leaves me stuck with float | None as the type hint for .secondary even though .secondary cannot possibly be None after __post_init__.
cast-ing self.secondary in __post_init__ doesn't work. This does:
NULL_FLOAT = float(int(uuid.uuid4())
@dataclasses.dataclass
class RelatedValues:
primary: float
_: dataclasses.KW_ONLY
secondary: float = NULL_FLOAT
def __post_init__(self):
if self.secondary == NULL_FLOAT:
self.secondary = self.primary
But it feels distinctly non-Pythonic.
This also works:
@dataclasses.dataclass
class RelatedValues:
primary: float
_: dataclasses.KW_ONLY
_secondary: float | None = None
def __post_init__(self):
if self._secondary is None:
self.secondary = self.primary
else:
self.secondary = self._secondary
or this:
@dataclasses.dataclass
class RelatedValues:
primary: float
_: dataclasses.KW_ONLY
_secondary: float | None = None
@property
def secondary(self) -> float:
if self._secondary is None:
self.secondary = self.primary
else:
self.secondary = self._secondary
But the latter two are just mangling my kwargs for the sake of type narrowing, which kind of feels wrong.
What am I missing?
答案1
得分: 1
如果 secondary 总是被赋予一个非 None 的值，那么在一开始就不应该将其类型定义为 float | None。实际上，只有作为 __init__ 方法的 参数 传递进来的 secondary_ 可能是 None。
from dataclasses import dataclass, field, InitVar
@dataclass
class RelatedValues:
primary: float
secondary: float = field(init=False)
secondary_: InitVar[float|None] = None
def __post_init__(self, secondary_):
if secondary_ is None:
secondary_ = self.primary
self.secondary = secondary_
只有在你一开始不想提供自己的 __init__ 方法时才需要上述步骤。例如，
@dataclass(init=False)
class RelatedValues:
primary: float
secondary: float
def __init__(self, primary: float, secondary: float = None):
if secondary is None:
secondary = primary
self.primary = primary
self.secondary = secondary
英文:
If secondary will always be assigned a non-None value, it should not be typed as float | None in the first place. It's only the argument to __init__, used to initialize self.secondary, that might be None.
from dataclasses import dataclass, field, InitVar
@dataclass
class RelatedValues:
primary: float
secondary: float = field(init=False)
secondary_: InitVar[float|None] = None
def __post_init__(self, secondary_):
if secondary_ is None:
secondary_ = self.primary
self.secondary = secondary_
These hoops are only necessary if you don't want to provide your own __init__ method in the first place. For example,
@dataclass(init=False)
class RelatedValues:
primary: float
secondary: float
def __init__(self, primary: float, secondary: float = None):
if secondary is None:
secondary = primary
self.primary = primary
self.secondary = secondary
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