Re: python3: 'where' keyword

From: AdSR (adsr_at_poczta.onet.pl)
Date: 01/07/05 

Date: Fri, 07 Jan 2005 21:04:28 +0100

Andrey Tatarinov wrote:
> Hi.
>
> It would be great to be able to reverse usage/definition parts in
> haskell-way with "where" keyword. Since Python 3 would miss lambda, that
> would be extremly useful for creating readable sources.
>
> Usage could be something like:
>
> >>> res = [ f(i) for i in objects ] where:
> >>> def f(x):
> >>> #do something

I don't know haskell, but it looks SQL-ish to me (only by loose
association). And it's not that unpythonic - it resembles

>>> res = [x for x in sequence if x.isOk()]

> or
>
> >>> print words[3], words[5] where:
> >>> words = input.split()

Here's a shorter version:

>>> print input.split()[3:5:2]

(Does it qualify as obfuscated Python code? :) )

> - defining variables in "where" block would restrict their visibility to
> one expression
>
> - it's more easy to read sources when you know which part you can skip,

Yes, I like the readability of it, too.

> compare to
>
> >>> def f(x):
> >>> #do something
> >>> res = [ f(i) for i in objects ]
>
> in this case you read definition of "f" before you know something about
> it usage.

When I first read your post, I thought "Well, just one more of those
Py3k ideas that appear on c.l.py every day." But as I look at the latter
example, I think you have just scratched my itch. The same thing has
bugged me more than once in my code.

I think this idea is of the same kind as the @decorator syntax. Guido
moved an operation to a point in the code where it was more visible. You
moved an operation to a more local context where (pun not intended) it
really belongs.

I'm usually rather conservative about Python syntax (@decorators,
absolute/relative imports, if-else operator), but this one could appear
in Python tomorrow and that would be too far in the future for me ;)

Cheers,

AdSR