[Python-Dev] An issue recently brought up in patch #872326 (generator expression) (original) (raw)

jiwon jiwon at softwise.co.kr
Sun Mar 21 21:00:20 EST 2004

IIRC, most people agreed on that iterables in the generator expression need to be pre-computed (including myself). But there's a recent issue about that.

First, if we precompute iterables in the generator expression, the behavior of it inevitably becomes different from generator expression. For instance, compare followings.

list((x,y) for x in iter('abcd') for y in iter('abcd')) [('a', 'a'), ('a', 'b'), ('a', 'c'), ('a', 'd')]

[(x,y) for x in iter('abcd') for y in iter('abcd')] [('a', 'a'), ('a', 'b'), ('a', 'c'), ('a', 'd'), ('b', 'a'), ('b', 'b'), ('b', 'c'), ('b', 'd'), ('c', 'a'), ('c', 'b'), ('c', 'c'), ('c', 'd'), ('d', 'a'), ('d', 'b'), ('d', 'c'), ('d', 'd')]

c.f)

a = iter("abcd"); b = iter("abcd"); [(x,y) for x in a for y in b] [('a', 'a'), ('a', 'b'), ('a', 'c'), ('a', 'd')]

Also, like arigo commented in the sf patch, the things we are looping over may depend on other stuff from the generator expression itself.

[x for l in [[1,2],[3,4]] for x in l] [1, 2, 3, 4] (y for m in [[1,2],[3,4]] for y in m) NameError: name 'm' is not defined

More comments are in sf, http://sourceforge.net/tracker/?func=detail&aid=872326&group_id=5470&atid=305470 Comments after 2004-03-19 22:37 (posted by quiver) are about that issue. Do we need to drop precomputation of iterables?

Regards, Jiwon.

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