Re: [PATCH v4 08/14] qapi/introspect.py: create a typed 'Annotated' data strutcure

[Eduardo Habkost <ehabkost@redhat.com>]

On Thu, Feb 04, 2021 at 04:37:45PM +0100, Markus Armbruster wrote:
> Eduardo Habkost <ehabkost@redhat.com> writes:
> 
> > On Wed, Feb 03, 2021 at 03:47:36PM +0100, Markus Armbruster wrote:
> >> John Snow <jsnow@redhat.com> writes:
> >> 
> >> > Presently, we use a tuple to attach a dict containing annotations
> >> > (comments and compile-time conditionals) to a tree node. This is
> >> > undesirable because dicts are difficult to strongly type; promoting it
> >> > to a real class allows us to name the values and types of the
> >> > annotations we are expecting.
> >> >
> >> > In terms of typing, the Annotated<T> type serves as a generic container
> >> > where the annotated node's type is preserved, allowing for greater
> >> > specificity than we'd be able to provide without a generic.
> >> >
> >> > Signed-off-by: John Snow <jsnow@redhat.com>
> > [...]
> >> > +class Annotated(Generic[_NodeT]):
> >> > +    """
> >> > +    Annotated generally contains a SchemaInfo-like type (as a dict),
> >> > +    But it also used to wrap comments/ifconds around scalar leaf values,
> >> > +    for the benefit of features and enums.
> >> > +    """
> >> > +    # Remove after 3.7 adds @dataclass:
> >> 
> >> Make this
> >> 
> >>        # TODO Remove after Python 3.7 ...
> >> 
> >> to give us a fighting chance to remember.
> >> 
> >> > +    # pylint: disable=too-few-public-methods
> >> > +    def __init__(self, value: _NodeT, ifcond: Iterable[str],
> >> > +                 comment: Optional[str] = None):
> >> 
> >> Why not simply value: _value?
> >
> > Example:
> >   x = C(1)
> >   y: C[int]
> >   y = C('x')  # mistake
> >
> > Declaring value as _NodeT does:
> > - Make the inferred type of x be Annotated[int].
> > - Catch the mistake above.
> 
> I smell overengineering.  I may well be wrong.

To me it's just regular and idiomatic use of Generic.

> 
> Without doubt, there are uses for using the type system for keeping
> SomeGenericType[SomeType] and SomeGenericType[AnotherType] apart.
> 
> But what do we gain by keeping the Annotated[T] for the possible T
> apart?

I understand this as (valid) criticism of the use of Generic.
If we don't want to make Generic[T1] and Generic[T2] be
different types, there's no point in using Generic at all.


> 
> _tree_to_qlit() doesn't care: it peels off the wrapper holding ifcond
> and comment, and recurses for the JSON so wrapped.  Regardless of what
> was wrapped, i.e. what kind of T we got.
> 
> Heck, it works just fine even if you wrap your JSON multiple times.  It
> doesn't give a hoot whether that makes sense.  Making sense is the
> caller's business.
> 
> So what does care?
> 
> Or am I simply confused?

Those are valid questions.  Maybe using Generic will be useful
in the future, but maybe we don't need it right now.

Personally, I don't care either way.  I just wish this small
choice don't became another obstacle for doing useful work.

> 
> 
> PS: As far as I can tell, _tree_to_qlit() doesn't give a hoot whether a
> dictionary's values are wrapped, either.

-- 
Eduardo