[Python-Dev] PEP 563: Postponed Evaluation of Annotations (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Wed Nov 8 20:49:01 EST 2017

On 8 November 2017 at 16:24, Guido van Rossum <guido at python.org> wrote:

I also don't like the idea that there's nothing you can do with a thunk besides calling it -- you can't meaningfully introspect it (not without building your own bytecode interpreter anyway).

Wait, that wasn't what I was suggesting at all - with thunks exposing their code object the same way a function does (i.e. as a __code__ attribute), the introspection functions in dis would still work on them, so you'd be able to look at things like which variable names they referenced, thus granting the caller complete control over how they resolved those variable names (by setting them in the local namespace passed to the call).

This is why they'd have interesting potential future use cases as general purpose callbacks - every local, nonlocal, global, and builtin name reference would implicitly be an optional parameter (or a required parameter if the name couldn't be resolved as a nonlocal, global, or builtin).

Using an AST instead of a string is also undesirable -- the AST changes in each release, and the usual strong compatibility guarantees don't apply here. And how are you going to do anything with it? If you've got a string and you want an AST node, it's one call away. But if you've got an AST node and you want either a string or the object to which that string would evaluate, you've got a lot of work to do. Plus the AST takes up a lot more space than the string, and we don't have a way to put an AST in a bytecode file. (And as Inada-san pointed out a thunk also takes up more space than a string.)

Nick, please don't try to save the thunk proposal by carefully dissecting every one of my objections. That will just prolong its demise.

Just the one objection, since you seem to be rejecting something I didn't suggest (i.e. adding an opaque callable type that the dis and inspect modules didn't understand). I agree that would be a bad idea, but it's also not what I was suggesting we do.

Instead, thunks would offer all the same introspection features as lambda expressions do, they'd just differ in the following ways:

That said, even without a full-fledged thunk based solution to handling lexical scoping I think there's a way to resolve the nested class problem in PEP 563 that works for both explicitly and implicitly quoted strings, while still leaving the door open to replacing implicitly quoted strings with thunks at a later date: stating that if users want such nested references to be resolvable at runtime, they need to inject a runtime reference to the outermost class into the inner class namespace.

That is, if you want to take:

class C:
    field = 1
    class D:
        def method(a: C.field):
            ...

and move it inside a function, that would actually look like:

def f():
    class C:
        field = 1
        class D:
            def method(a: C.field):
                ...
    C.D.C = C # Make annotations work at runtime
    return f

That leaves the door open to a future PEP that proposes thunk-based annotations as part of proposing thunks as a new low level delayed evaluation primitive.

Cheers, Nick.

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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