[Python-ideas] Re: [dataclasses] add a NON_FIELDS sentinel after which all attributes are ignored.

[Eric V. Smith via Python-ideas]

On 6/23/2023 11:34 AM, Joao S. O. Bueno wrote:



On Fri, Jun 23, 2023 at 12:18 PM Eric V. Smith  wrote:



On Jun 23, 2023, at 9:34 AM, Joao S. O. Bueno 
wrote:




On Fri, Jun 23, 2023 at 2:35 AM Jelle Zijlstra
 wrote:



El jue, 22 jun 2023 a las 8:22, Randolf Scholz
() 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.

I think you can make dataclasses itself ignore the field by judicious 
use of `field` parameters. If there's some code that's looking through 
`dataclasses.fields` but wants to ignore some fields, irrespective of 
what dataclasses is doing, then I'd say it's on the caller to have a 
list of fields to ignore.


But without knowing the use case, it's hard to say.

Eric

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[Python-ideas] Re: [dataclasses] add a NON_FIELDS sentinel after which all attributes are ignored.

[Eric V. Smith via Python-ideas]

On Jun 23, 2023, at 9:34 AM, Joao S. O. Bueno  wrote:On Fri, Jun 23, 2023 at 2:35 AM Jelle Zijlstra  wrote:El jue, 22 jun 2023 a las 8:22, Randolf Scholz () 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?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#76533091I like the dataclasses.attribute idea, though - (but it will also require static type checking tools to review theirdataclass 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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[Python-ideas] Re: [dataclasses] add a NON_FIELDS sentinel after which all attributes are ignored.

[Joao S. O. Bueno]

On Fri, Jun 23, 2023 at 12:18 PM Eric V. Smith  wrote:

> 
>
> On Jun 23, 2023, at 9:34 AM, Joao S. O. Bueno  wrote:
>
> 
>
>
> On Fri, Jun 23, 2023 at 2:35 AM Jelle Zijlstra 
> wrote:
>
>>
>>
>> El jue, 22 jun 2023 a las 8:22, Randolf Scholz ()
>> 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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[Python-ideas] Re: [dataclasses] add a NON_FIELDS sentinel after which all attributes are ignored.

[Thiago Bellini Ribeiro]

A possible suggestion for your problem:

On strawberry-graphql we have a solution for that, by annotating the field
with "strawberry.Private"[1], like this:

  @strawberry.type
  class Foo:
  str_attr: str
  str_private_attr: Private[str]

That Private is defined as:

  Private = Annotated[T, StrawberryPrivate()]

Meaning we could also do "str_private_attr: Annotated[str,
StrawberryPrivate()]"

When introspecting the dataclass fields to generate the graphql schema, we
will check if the annotation is annotated, and if it is and contains a
"StrawberryPrivate()" instance in its __args__, we will exclude that field
from the graphql API.

[1]
https://github.com/strawberry-graphql/strawberry/blob/main/strawberry/private.py

On Fri, Jun 23, 2023 at 10:32 AM Joao S. O. Bueno  wrote:

>
>
> On Fri, Jun 23, 2023 at 2:35 AM Jelle Zijlstra 
> wrote:
>
>>
>>
>> El jue, 22 jun 2023 a las 8:22, Randolf Scholz ()
>> 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)
>>>
>>> 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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-- 
Thiago Bellini Ribeiro | https://bellini.dev
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[Python-ideas] Re: [dataclasses] add a NON_FIELDS sentinel after which all attributes are ignored.

[Joao S. O. Bueno]

On Fri, Jun 23, 2023 at 2:35 AM Jelle Zijlstra 
wrote:

>
>
> El jue, 22 jun 2023 a las 8:22, Randolf Scholz ()
> 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)
>>
>> 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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[Python-ideas] Re: [dataclasses] add a NON_FIELDS sentinel after which all attributes are ignored.

[Jelle Zijlstra]

El jue, 22 jun 2023 a las 8:22, Randolf Scholz ()
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)
>
> 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)`?


> ```
>
> 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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[Python-ideas] Re: [dataclasses] add a NON_FIELDS sentinel after which all attributes are ignored.

[Christopher Barker]

Hmm,

It does make you wonder if using the presence of an annotation to indicate
a field was a good choice -- but here we are.

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

Better yet, a dataclasses.attribute object that would indicate that:

from dataclasses import dataclass, field, attribute
@dataclass
class C:
x: int = 0
mylist: list[int] = field(default_factory=list)
y: int = attribute(value=0)

x and mylist are fields, y is not. y would get set to the value specified.

Then the type annotation is left unadulterated -- much easier for the type
checkers.

-CHB



-- 
Christopher Barker, PhD (Chris)

Python Language Consulting
  - Teaching
  - Scientific Software Development
  - Desktop GUI and Web Development
  - wxPython, numpy, scipy, Cython
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