What is the proper way to use descriptors as fields in Python dataclasses?

user_16592571 0
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I've been playing around with python dataclasses and was wondering: What is the most elegant or most pythonic way to make one or some of the fields descriptors?

In the example below I define a Vector2D class that should be compared on its length.

from dataclasses import dataclass, field
from math import sqrt

@dataclass(order=True)
class Vector2D:
    x: int = field(compare=False)
    y: int = field(compare=False)
    length: int = field(init=False)
    
    def __post_init__(self):
        type(self).length = property(lambda s: sqrt(s.x**2+s.y**2))

Vector2D(3,4) > Vector2D(4,1) # True

While this code works, it touches the class every time an instance is created, is there a more readable / less hacky / more intended way to use dataclasses and descriptors together?

Just having length as a property and not a field will work but this means I have to write __lt__, et.al. by myself.

Another solution I found is equally unappealing:

@dataclass(order=True)
class Vector2D:
    x: int = field(compare=False)
    y: int = field(compare=False)
    length: int = field(init=False)
    
    @property
    def length(self):
        return sqrt(self.x**2+self.y**2)
    
    @length.setter
    def length(self, value):
        pass

Introducing a no-op setter is necessary as apparently the dataclass-created init method tries to assign to length even though there isn't a default value and it explicitly sets init=False...

Surely there has to be a better way right?

python-dataclasses python-descriptors python
2 Answers

Might not answer your exact question, but you mentioned that the reason that you didnt want to have length as a property and a not field was because you would have to

write __lt__, et.al by myself

While you do have to implement __lt__ by yourself, you can actually get away with implementing just that

from functools import total_ordering
from dataclasses import dataclass, field
from math import sqrt

@total_ordering
@dataclass
class Vector2D:
    x: int
    y: int

    @property
    def length(self):
        return sqrt(self.x ** 2 + self.y ** 2)

    def __lt__(self, other):
        if not isinstance(other, Vector2D):
            return NotImplemented

        return self.length < other.length

    def __eq__(self, other):
        if not isinstance(other, Vector2D):
            return NotImplemented

        return self.length == other.length


print(Vector2D(3, 4) > Vector2D(4, 1))

The reason this works is because total_ordering just adds all the other equality methods based on __eq__ and __lt__

user_36450160

I do not think that the example you present is a good use case for what you are trying to do. Nevertheless, for completeness sake, here is a possible solution to your problem:

@dataclass(order=True)
class Vector2D:
    x: int = field(compare=False)
    y: int = field(compare=False)
    length: int = field(default=property(lambda s: sqrt(s.x**2+s.y**2)), init=False)

This works because dataclass sets defaults as values on the class attributes unless the value is a list, dict or set.

user_171832740
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