[Python-ideas] Where-statement (Proposal for function expressions)

[Mike Meyer]

On Fri, 17 Jul 2009 14:02:34 +1200
Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:

> Daniel Stutzbach wrote:
> 
> > If the
> > left-hand side were in the where-block's scope, then:
> > 
> > x = 0
> > x = i where:
> >    i = 5
> > print x
> > 
> > Would print "0" not "5".
> 
> My intuitive expectations are different for assignment
> to a bare name vs. assignment to an item or attribute.
> In
> 
>    x = i where:
>      i = 5
> 
> I would expect x to be bound in the current scope,
> not the where-block scope. But I would expect
> 
>    x[i] = 5 where:
>      x = y
>      i = 7
> 
> to be equivalent to
> 
>    y[7] = 5
> 
> i.e. both x and i are *evaluated* in the where-block
> scope.
> 
> I'm not sure where this leaves us with respect to
> augmented assignment, though. If you follow it to
> its logical conclusion, then
> 
>    x += i where:
>      i = 5
> 
> should be equivalent to
> 
>    x = x.__iadd__(i) where:
>      i = 5
> 
> and then the evaluation and rebinding of 'x' would
> happen in different scopes!

That evaluating an expression to the right of an augmented assignment
might return something different than what you would bind that
expression to has always been an issue with augmented assignments. I
believe this particular problem belongs has more to do with them than
with the proposed where statement.

I think the intuitive behavior doesn't involve position relative to
the assignment, but binding vs. non-bindings. What you *expect* is
that expressions left of the where will be evaluated in the inner
scope (created by the where), but that any bindings that happen there
will happen in the outer scope (before the where).

That makes what you want to happen happen - *most* of the time:

i = 3
x += i where:
   i = 5

increments x by 5.

i = 'a'
x[i] = 'b' where:
   i = 'c'

sets x['c'] to 'b'.

Things get really ugly when the bound variables start appearing in
both scopes combined with augmented assignments:

x = 3
x += 4 where:
    x = 5
print x

What should this print?

     <mike
-- 
Mike Meyer <mwm at mired.org>		http://www.mired.org/consulting.html
Independent Network/Unix/Perforce consultant, email for more information.

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