[Python-Dev] zip() and list-comprehension with commas

[Ka-Ping Yee]

On Tue, 11 Jul 2000, Paul Prescod wrote:
> 
> Parallel iteration and list comprehensions are separate. I do prefer a
> parallel() (merge?) builtin to the semicolon syntax, myself. You could
> even define xparallel which is the lazy version (as in xrange).

I like this idea.  This whole parallel iteration thing could be solved
with a single built-in and no syntax changes.

    for x, y in parallel([10, 20, 30], [1, 2]):

Or "zip", since you're zipping the lists together (i foresee potential
geometric or concurrency-related interpretations of "parallel"):

    for x, y in zip([10, 20, 30], [1, 2]):

    [(x, y+z), for (x, y) in zip((1, 2, 3, 4, 5, 6), "abc"), for z in "de"]


Taking this all together provides one coherent solution:

    - zip() built-in produces a lazy list-zipper object

    - list-comps are written [<expr>, <clause>, <clause>, ...]
          where each <clause> is "for x in y" or "if z"

Advantages:

    - no semicolons; semicolons remain solely statement separators

    - no possible confusion between parallel and nested iteration

    - comma remains the top-level separator within lists, so
          grouping of list-comp clauses is clear

    - dict-comp is possible and straightforward if we really want it

    - looks much prettier than map/lambda :)

How's that for a serious proposal?



-- ?!ng