Fredrik Lundh wrote: > Dustan wrote: > > >>>> foo =\ > > [[[1,2,3],[4,5,6],[7,8,9]], > > [[3,2,1],[6,5,4],[9,8,7]]] > > > > Here, foo appears to be a 3-dimensional list - except it's supposed to > > be 2-dimensional. The inner-list-of-lists is a result of how I'm > > producing the data, and now I want to do a mass-concatenation (or > > extending) of the inner-list-of-lists, and come up with this result: > > > >>>> foo == [[1,2,3,4,5,6,7,8,9],[3,2,1,6,5,4,9,8,7]] > > True > > that still doesn't explain your "the expression must be used in a list > comprehension" requirement, though.
Oh, right; sorry about the confusion. The list isn't quite that simple, and in order to pull the right pieces together, I use a list comprehension. > assuming that the sizes are > varying, and at least sometimes a lot larger than 3x3, I'd probably > write the above as > > for index, item in enumerate(foo): > this = [] > for i in item: > this.extend(i) > foo[index] = this I'll see if that works. > which should be pretty efficient, since it avoids unnecessary function > calls, and is amortized linear time instead of O(N**2). > > or, if I was in a hurry, and didn't really care if the inner sequences > were lists or tuples: > > foo = map(Tkinter._flatten, foo) > > </F> Steven D'Aprano wrote: > I don't know if this is the best, but it didn't take long to come up with > it: > > >>> foo = [[[1,2,3],[4,5,6],[7,8,9]], > ... [[3,2,1],[6,5,4],[9,8,7]]] > >>> > >>> def unroll(list3d): > ... newl = [] > ... for sublist in list3d: > ... newl.append(sum(sublist, [])) > ... return newl deja vu... > >>> bar = unroll(foo) > >>> bar > [[1, 2, 3, 4, 5, 6, 7, 8, 9], [3, 2, 1, 6, 5, 4, 9, 8, 7]] > > > Repeat after me: > > "Not everything has to be a one-liner." Not everything has to be a one-liner. But readability helps. > If sum() is too slow, because your sub-lists are huge, you can easily > factor that out and replace it with something using extend. > > > -- > Steven. -- http://mail.python.org/mailman/listinfo/python-list
