[Python-Dev] PEP 526 ready for review: Syntax for Variable and Attribute Annotations

[Guido van Rossum]

On Tuesday, August 30, 2016, Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 31 August 2016 at 13:37, Jack Diederich <jackdied at gmail.com
> <javascript:;>> wrote:
> > On Tue, Aug 30, 2016 at 11:03 PM, Guido van Rossum <guido at python.org
> <javascript:;>> wrote:
> >> But myfunc.__annotations__ already exists -- PEP 3107 puts the
> >> signature annotations there. The problem with co_annotations is that
> >> annotations are evaluated (they can be quite complex expressions, e.g.
> >> Optional[Tuple[int, int, some_mod.SomeClass]]), while co_varnames is
> >> just a list of strings. And code objects must be immutable. The issue
> >> with rejecting duplicate annotations so sternly is the same as for the
> >> previous bullet.
> >>
> >
> > If we disallow re-assignment of types as a syntax error then the conflict
> > with myfunc.__annotations__ goes away for vars that share a name with the
> > function arguments. The fact that variables with types can't be known
> until
> > the function body executes a particular line is .. I'm not sure how to
> deal
> > with that. For modules and classes you can assert that the body at the
> top
> > indent level has been executed. For functions you can only assert that it
> > has been parsed. So myfunc.__annotations__ could say that the type has a
> > definition but only later know what the definition is.
>
> What if we included local variable annotations in func.__annotations__
> as cells, like the entries in func.__closure__?
>
> We could also use that as a micro-optimisation technique: once the
> type annotation cell is populated, CPython would just use it, rather
> than re-evaluating the local variable type annotation expression every
> time the function is called.
>

But what runtime use have the annotations on locals? They are not part of
any inspectable interface. I don't want to spend any effort on them at
runtime. (Just the bit that they are treated as locals.)

--Guido


-- 
--Guido (mobile)
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