list comprehensions whats happening here

[Rainer Deyke]

"Carlos Ribeiro" <cribeiro at mail.inet.com.br> wrote in message
news:mailman.987166938.10180.python-list at python.org...
> I also lost. I did a small change on your example to better illuminate
what
> is happening:
>
>  >>> [[i,j] for i in range(3), for j in 'abc']
> [[[0, 1, 2], 'a'], [[0, 1, 2], 'b'], [[0, 1, 2], 'c']]
>  >>> [[i,j] for i in range(3), for j in 'abc',]
> [[[0, 1, 2], 'abc']]
>  >>> [[i,j] for i in range(3) for j in 'abc']
> [[0, 'a'], [0, 'b'], [0, 'c'], [1, 'a'], [1, 'b'], [1, 'c'], [2, 'a'], [2,
> 'b'], [2, 'c']]
>
> And also, a fourth case for completeness:
>
>  >>> [[i,j] for i in range(3) for j in 'abc',]
> [[0, 'abc'], [1, 'abc'], [2, 'abc']]
>
> So it seems that the idiom
>
>    for i in range(x),
>
> with the comma right after the for clause, "reduces" the iterated list to
> the original list again, and then apply it to all further iterations of
the
> list comprehension. Now, I don't know if this is by design or by accident;
> all I know is that this is something that may be exploited as a useful
> idiom. For instance, neither map or zip have this behavior, of applying
the
> same list to all members of the other.

Hint: '[1,2,3],' is a tuple of 1.


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Rainer Deyke (root at rainerdeyke.com)
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