On Fri, Jun 23, 2023 at 12:18 PM Eric V. Smith <eric@trueblade.com> wrote:

On Jun 23, 2023, at 9:34 AM, Joao S. O. Bueno <gwidion@gmail.com> wrote:




On Fri, Jun 23, 2023 at 2:35 AM Jelle Zijlstra <jelle.zijlstra@gmail.com> wrote:


El jue, 22 jun 2023 a las 8:22, Randolf Scholz (<randolf.scholz@gmail.com>) escribió:
Dataclasses should provide a way to ignore a type hinted attributes, and not consider them as fields.

For example, some attributes might be derived during `__post_init__` from the values of the fields or other variables.

If one wants to still type hint these attributes, one has to awkward workarounds to avoid having dataclass interpret them as fields. (https://stackoverflow.com/questions/76532816)

But it’s not clear (to me) why not being a field is desirable. Why is it important?

Can't know  - not my design, it was Randolf's question.
They might represent an internal state that should not be relayed on serialization or conversion, for example.
I can imagine some scenarios where I'd want some instance attributes to be shorter lived and non-transient,
although, I'd more likely build the class "manually" instead of a dataclass - or, more likely, put the dataclass under
a wrapper layer that would handle the "perishable" states.



Eric


I propose `NON_FIELDS` sentinel, analogous to `KW_ONLY`. (alternative name suggestions welcome). when writing a dataclass, all attributes after this sentinel are ignored and not considered fields.

```
@dataclass
class Foo:
    field0: int
    field1: int

    _: KW_ONLY

   fieldN: int

    _: NON_FIELDS

    attr0: int   # @dataclass will ignore this type hint.

How is this different from `attr0: int = field(init=False)`?

attr0 would be listed as a `field` in the introspectable attributes of the dataclass in this way.
That is why I did not suggest that in my initial answer to Randolf on stackoverflow:
https://stackoverflow.com/questions/76532816/type-hint-extra-attributes-not-fields/76533091#76533091

I like the dataclasses.attribute idea, though - (but it will also require static type checking tools to review their
dataclass special casing - it looks like there is no escape from that). 

 
 
```

Additionally one could consider adding an `attribute` typing construct, such that `attr0: attribute[int]` would mark it as a non-field attribute.
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