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list-display semantics?

Emile van Sebille emile at fenx.com
Sun Jun 10 15:21:46 EDT 2001 
I don't quite get the results you get, but from:
print [[x,y] for x in [1,2,3] for y in [4,5,6]]
I get
[[1, 4], [1, 5], [1, 6], [2, 4], [2, 5], [2, 6], [3, 4], [3, 5], [3, 6]]

Which looks like what you want.

From:
print [x for x in [1, 2, 3], y for y in [4, 5, 6]]
I get:
[[1, 2, 3], [1, 2, 3], [1, 2, 3], 6, 6, 6]

The difference is that the comma following [1,2,3] causes [1,2,3] to be
treated as the first element of a tuple, and the then current value of y as
the second element (in my case 6, in your case 9).  Now your for y in
[4,5,6] might as well be for i in range(3) as the y value in the loop is not
used in the result.  Note the subtle namespace issue.  Your line of code
then reads: "Give me a list of x's for x iterating over ([1,2,3] , y) three
times.

HTH,

--

Emile van Sebille
emile at fenx.com

---------
"jainweiwu" <parywu at seed.net.tw> wrote in message
news:9g0dhl$if1$1 at news.nsysu.edu.tw...
> 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
>
>
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