[Python-ideas] New 3.x restriction in list comprehensions

[Guido van Rossum]

On Fri, Sep 17, 2010 at 12:44 PM, Raymond Hettinger
<raymond.hettinger at gmail.com> wrote:
> In Python2, you can transform:
>
>  r = []
>  for x in 2, 4, 6:
>       r.append(x*x+1)
>
> into:
>
>   r = [x*x+1 for x in 2, 4, 6]
>
> In Python3, the first still works but the second gives a SyntaxError.
> It wants the 2, 4, 6 to have parentheses.
>
> The good parts of the change:
>  + it matches what genexps do
>  + that simplifies the grammar a bit (listcomps bodies and genexp bodies)
>  + a listcomp can be reliably transformed to a genexp
>
> The bad parts:
>  + The restriction wasn't necessary (we could undo it)
>  + It makes 2-to-3 conversion a bit harder
>  + It no longer parallels other paren-free tuple constructions:
>        return x, y
>        yield x, y
>        t = x, y
>           ...
>  + It particular, it no longer parallels regular for-loop syntax
>
> The last part is the one that seems the most problematic.
> If you write for-loops day in and day out with the unrestricted
> syntax, you (or least me) will tend to do the wrong thing when
> writing a list comprehension.  It is a bit jarring to get the SyntaxError
> when the code looks correct -- it took me a bit of fiddling to figure-out
> what was going on.
>
> My question for the group is whether it would be a good
> idea to drop the new restriction.

This was intentional. It parallels genexps and it avoids an ambiguity
(for the human reader -- I know the parser has no problem with it :-).

Please don't change this back. (It would violate the moratorium too...)

-- 
--Guido van Rossum (python.org/~guido)