list-display semantics?
Bengt Richter
bokr at accessone.com
Sun Jun 10 17:54:27 EDT 2001
On Mon, 11 Jun 2001 02:29:47 +0800, "jainweiwu" <parywu at seed.net.tw>
wrote:
>Hi all:
>I tried the one-line command in a interaction mode:
>[x for x in [1, 2, 3], y for y in [4, 5, 6]]
>and the result surprised me, that is:
>[[1,2,3],[1,2,3],[1,2,3],9,9,9]
>Who can explain the behavior?
>Since I expected the result should be:
>[[1,4],[1,5],[1,6],[2,4],...]
>--
>Pary All Rough Yet.
>parywu at seed.net.tw
>
>
I think the first comma made your expression legal
and equivalent to
[x for x in ([1, 2, 3], y) for y in [4, 5, 6]]
so x took on two values: [1, 2, 3] and the incoming value
of y, which happened to be bound to a left-over 9 from
some previous activity, because the bindings for the elements
of the sequence ([1, 2, 3], y) were set up before the y loop
got started. So the loop y sequenced 4-5-6 for for three elements
of [1, 2, 3] and then x took on the value of y from the tuple
which hadn't changed it was created, hence the 9, for three more
elements of the final result. The actual values for the y loop
make no difference except that the last will be available next
time. Do a del y and you will get a NameError exception if you
try to re-run your statement.
Since you used y for the for expression, if you ran the
expression again, you'd get 6's in the place of the 9's.
>>> y='y'
>>> [x for x in [1, 2, 3], y for y in [4, 5, 6]]
[[1, 2, 3], [1, 2, 3], [1, 2, 3], 'y', 'y', 'y']
>>> [x for x in [1, 2, 3], y for y in [4, 5, 6]]
[[1, 2, 3], [1, 2, 3], [1, 2, 3], 6, 6, 6]
Then note the 6 left over from the first go above.
Try the following (with [4,5,6]) to get what you expected.
(I left off the 6 to avoid line wrap here :)
>>> [[x,y] for x in[1,2,3] for y in [4,5]]
[[1, 4], [1, 5], [2, 4], [2, 5], [3, 4], [3, 5]]
(caveat lector: I am neither Latinist nor Python guru
yet ;-)
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