[Python-ideas] back-references in comprehensions

[Jacob Holm]

On 2011-10-08 19:48, yoch melka wrote:
> I would like to use backreferences in list comprehensions (or other
> comprehensions), such as :
> 
> [[elt for elt in lst if elt] for lst in matrix if \{1}]
> # \{1} is back-reference to filter result of the
> # first comprehension ([elt for elt in lst if elt])
> 
> It would be possible to do this ?

I don't think the syntax you propose is going to fly, but a similar
feature has been discussed before on this list.  I prefer this version:

[[elt for elt in lst if elt] as r for lst in matrix if r]

But I don't have much hope that it will ever get in.

FWIW, the example can already be written in several ways in current
python, e.g.:

1) [r for lst in matrix for r in ([elt for elt in lst if elt],) if r]
2) filter(None, ([elt for elt in lst if elt] for lst in matrix))

HTH

- Jacob