[Python-Dev] once [was: Simple Switch statementZ]

[Jim Jewett]

On 6/28/06, Guido van Rossum <guido at python.org> wrote:

>   def index_functions(n):
>     return [(lambda i=i: i) for i in range(n)]

> which works but has the disadvantage of returning a list of functions
> of 0 or 1 argument

> I believe at least one poster has pointed out that 'once' (if defined
> suitably) could be used as a better way to do this:

Cleaner, yes.  But you would still have to remember the once, just as
you have to remember the i=i, so I don't think it would actually save
any confusion in practice.

Another alternative might be letting functions get at themselves,
rather than just their names.  (Methods can save attributes on self,
but functions are out of luck if someone else reused their name.)

> Perhaps 'once' is too misleading a name, given the confusion you
> alluded to earlier. Maybe we could use 'capture' instead? A capture
> expression would be captured at every function definition time,
> period.

I think it would have the same problem; I would still want to read
that as "The first time you run this, capture the result.", rather
than "Capture the binding current at funcdef time, even though you're
skipping all the other statements at this indent level."

> Capture expressions outside functions would be illegal or
> limited to compile-time constant expressions (unless someone has a
> better idea).

At a minimum, it should be able to capture the expression's current
value at load-time, which might well involve names imported from
another module.

> A capture expression inside "if 0:" would still be
> captured to simplify the semantics (unless the compiler can prove that
> it has absolutely no side effects).

Running code that was guarded by "if 0:" sounds like a really bad idea.

-jJ