How to create a non-field instance-level variable on a dataclass?

Question

If I understand well, ClassVar is a way of creating a class-variable on a dataclass, a variable which won't be considered as a field. So, for example:

@dataclass
class MyData:
    name: str
    age: int
    parent: ClassVar[Optional["MyData"]] = None

jake_data = MyData(name="Jake", age=34)

dataclasses.fields(jake_data) # This will output only two fields, as status is a ClassVar

But if I want to modify parent at the instance level after the instance's initialisation, it should not be a ClassVar, as PEP 526 makes it clear that the ClassVar annotation should only be used for variables which are not replaced at instance level. And I don't want to declare parent as a field with a default value because then it would be... a dataclass field!

For the context: I use libraries that rely on the output of dataclasses.fields, that's why I want it to be a non-field variable.

Is there a pythonic and readable way to do that?

There are init only varaibles, which don't seem to be what you want. Dataclasses aren't really designed to have non-transient variables that aren't fields. How would you be using the status? — Patrick Haugh
– Patrick Haugh, Commented Feb 24, 2020 at 19:03
@PatrickHaugh, I've edited the question (replaced status by parent) so it is closer to the use I have. As you can see, I am using this variable as a reference. I will need to set it only once, but it needs to be after initialisation. — Jundiaius
– Jundiaius, Commented Feb 24, 2020 at 19:09

Jundiaius · Accepted Answer · 2020-03-13 10:57:11Z

5

Just don't annotate it, and define it in the __post_init__, so something like:

@dataclass
class MyData:
    name: str
    age: int

    def __post_init__(self):
        self.parent = None
        # whatever else

edited Mar 13, 2020 at 10:57

Jundiaius

8,0085 gold badges39 silver badges49 bronze badges

answered Feb 24, 2020 at 19:05

juanpa.arrivillaga

97.6k14 gold badges141 silver badges190 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

How to create a non-field instance-level variable on a dataclass?

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related