[Python-ideas] Introducing where clauses

[Jim Jewett]

On Mon, Jun 22, 2009 at 8:46 AM, Andrey Popp<8mayday at gmail.com> wrote:
>    [(y, y) for x in some_iterable if y < 2 where y = f(x)]
>
> looks more naturally than three suggested variants.

Only if you are already thinking about SQL.

An "as" would be almost as good as a "where", and would better match
the way python has evolved so far.

But that still doesn't answer the real question.  In isolation, I
would see either as a real (if perhaps small and isolated) improvement
to readability.

The catch is that the improvement must be dramatic enough to justify
the cost -- which is extra complexity to the language as a whole.
That isn't a cost you see as easily, because you're thinking about
comprehensions; it is instead a small tax paid by people doing regular
function calls and if branching and for loops.  It isn't a big tax,
but it is cumulative, and python has worked hard to minimize it.

-jJ