[Python-Dev] Possible resolution of generator expression variable capture dilemma

[Guido van Rossum]

> However, I'm suggesting something different. I'm *not* proposing to
> make it *local* to the loop -- its name will still reside in the
> enclosing namespace, and its value will still be available after the
> loop finishes. In fact, if it's not referenced from any nested scope,
> there will be no change in semantics at all.
> 
> What *will* change is perhaps best explained by means of the
> implementation, which is very simple. If the loop variable is
> referenced from a nested scope, it will be held in a cell. Now, on
> each iteration, instead of replacing the contents of the cell as a
> normal assignment would, we create a *new* cell and re-bind the name
> to the new cell.

If I had known Scheme 15 years ago I might have considered this -- it
is certainly an interesting approach.  I'll have to ponder this a lot
more before accepting it now; the potential consequences seem
enormous.  (It also pretty much forces this particular way of
implementing nested scopes.)

I think it's definitely out of the question before 3.0; it's too
subtle a change to too fundamental a construct.

--Guido van Rossum (home page: http://www.python.org/~guido/)