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Sun Mar 21 21:00:20 EST 2004 · https://mail.python.org/pipermail/python-dev/2004-March/043396.html

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