[Python-Dev] accumulator display syntax

[Michael Hudson]

Guido van Rossum <guido at python.org> writes:

>> On Monday 20 October 2003 04:30 pm, Guido van Rossum wrote:
>> > > We are indeed sure (sadly) that list comprehensions leak control variable
>> > > names.
>> >
>> > But they shouldn't.  It can be fixed by renaming them (e.g. numeric
>> > names with a leading dot).
>> 
>> Hmmm, sorry?
>> 
>> >>> [.2 for .2 in range(3)]
>> SyntaxError: can't assign to literal
>> 
>> I think I don't understand what you mean.
>
> I meant that the compiler should rename it.

Implementing this might be entertaining.  In particular what happens
if the iteration variable is a local in the frame anyway?  I presume
that would inhibit the renaming, but then there's a potentially
confusing dichotomy as to whether renaming gets done.  Of course
you could *always* rename, but then code like 

def f(x):
    r = [x+1 for x in range(x)]
    return r, x

becomes even more incomprehensible (and changes in behaviour).

And what about horrors like

    [([x for x in range(10)],x) for x in range(10)]

vs:

    [([x for x in range(10)],y) for y in range(10)]

?

I suppose you could make a case for throwing out (or warning about)
all these cases at compile time, but that would require significant
effort as well (I think).

Cheers,
mwh

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