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Tue Oct 21 13:24:25 EDT 2003 · https://mail.python.org/pipermail/python-dev/2003-October/039155.html

On Tuesday 21 October 2003 06:53 pm, Guido van Rossum wrote:
   ...
> > maybe an optimization IS possible if all the user code does with the
> > LC is loop on it (or anyway just get its iter(...) and nothing else).
>
> But that's not very common, so I don't see the point of putting in the

It IS common, at least in the code I write, e.g.:

  d = dict([ (f(a), g(a)) for a in S ])

  s = sets.Set([ a*a for a in S ])

  totsq = sum([ x*x for x in S ])

etc.  I detest the look of those ([ ... ]), but that's the closest I get to 
dict comprehensions, set comprehensions, etc.

> except when it doesn't, and then making the list comprehension lazy
> can be a mistake: the following example
>
>   for key in [k for k in d if d[k] is None]:
>        del d[key]
>
> is *not* the same as
>
>   for key in d:
>       if d[key] is None:
>           del d

Well, no, but even if that last statement was "del d[key]" you'd still be
right:-).  Even in a situation where the list comp is only looped over once,
code MIGHT still be relying on the LC having "snapshotted" and/or
exhausted iterators IT uses.  I was basically thinking of passing the LC
as argument to something -- the typical cases where I use LC now and
WISH they were lazy, as above -- rather about for loops.  And even
when the LC _is_ an argument there might be cases where its current
strict (nonlazy) semantics are necessary.  Oh well!

Alex

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[Python-Dev] accumulator display syntax