Re: [PATCH v2 4/4] doc: bitbake-user-manual-metadata: move inline python examples to inherit section

Hi Dawid,

On 5/1/26 11:14 AM, Dawid Bijak wrote:
> The two inline Python examples,
> 
>    inherit_defer ${@'classname' if condition else ''}
>    inherit_defer ${@bb.utils.contains('VARIABLE', 'something', 'classname', '', d)}
> 
> are presented as inherit_defer-specific techniques, but they work
> with plain inherit too. Their placement under
> inherit_defer suggests deferred evaluation is required, which is
> not the case: the ${@...} expression is evaluated when the
> inherit line is parsed in both forms.
> 
> Move the inline Python expression examples from the inherit_defer section
> up to the inherit section, since they apply to both directives.
> 

As said in V1 and in this form: NACK.

What's the raison d'être of inherit_defer if inherit "works just as well"?

To quote Richard in v1:

"""
It works, as long as you are sure that COND won't be changed after the
inherit. When mutliple files altering the variables are involved, that
is often unclear. I think the doc's intent is therefore to recommend
anything with variable accesses is therefore deferred, unless the user
is sure they know what they're doing.
"""

The commit introducing inherit_defer in BitBake is also pretty explicit 
why it was added, see 5c2e840eafeb ("ast/BBHandler: Add inherit_defer
support").

So it may appear to work for you in your specific usecase, but it 
doesn't always. We are very careful to not add unnecessary complexity, 
be it operators or directives so I trust Richard to not have merged 
something that does nothing that isn't already supported by another 
directive or operator. Yes, inherit_defer is useful for the native class 
which we always want to be last, but it's not why this directive was 
added in the first place, it's only piggy-backing there.

So, as said in v1, we are not using footguns as examples. We can however 
say that something may appear to work but is a footgun, as suggested in 
v1 where I said "You can provide an example if you really want to in the 
inherit section, and add a big fat warning after it that it'll break in 
some scenario so you really shouldn't do that and instead use 
inherit_defer.".

Cheers,
Quentin