[Python-ideas] Explicit variable capture list

[Guido van Rossum]

On Wed, Jan 20, 2016 at 4:10 PM, Steven D'Aprano <steve at pearwood.info>
wrote:

> I'm just tossing the "static block" idea out for discussion, but if you
> want a justification here are two differences between capture/static
> and global/nonlocal which suggest they aren't that similar and so we
> shouldn't feel obliged to use the same syntax.
>
> (1) global and nonlocal operate on *names*, not values. E.g. after
> "global x", x refers to a name in the global scope, not the local scope.
>
> But "capture"/"static" doesn't affect the name, or the scope that x
> belongs to. x is still a local, it just gets pre-initialised to the
> value of x in the enclosing scope. That makes it more of a binding
> operation or assignment than a declaration.
>
> (2) If we limit this to only capturing the same name, then we can only
> write (say) "static x", and that does look like a declaration. But maybe
> we want to allow the local name to differ from the global name:
>
>     static x = y
>
> or even arbitrary expressions on the right:
>
>     static x = x + 1
>
> Now that starts to look more like it should be in a block of code,
> especially if you have a lot of them:
>
>     static x = x + 1
>     static len = len
>     static data = open("data.txt").read()
>
> versus:
>
>     static:
>         x = x + 1
>         len = len
>         data = open("data.txt").read()
>
>
> I acknowledge that this goes beyond what the OP asked for, and I think
> that YAGNI is a reasonable response to the static block idea. I'm not
> going to champion it any further unless there's a bunch of interest from
> others.


Yeah, your arguments why it's different from global/nonlocal are
reasonable, but the question remains whether we really need all that
functionality. IIRC C++ lambdas only allow capturing a variable's value,
not an expression's.

So we should ask ourselves first: if we *only* had some directive that
captures some variables' values, essentially like the len=len argument
trick but without affecting the signature (e.g. just "static x, y, z"), how
much of the current pain would be addressed, and how much would remain?


> (I'm saving my energy for Eiffel-like require/ensure blocks
> *wink*).
>

Now you're making me curious.

-- 
--Guido van Rossum (python.org/~guido)
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