Rewriting Recipe/410687 without map and lambda

[sofeng]

On Dec 11, 8:29 pm, "Terry Reedy" <tjre... at udel.edu> wrote:
> "sofeng" <sofeng... at gmail.com> wrote in message
>
> news:ea3c295f-fc27-40a1-a903-7504dd45ea6f at s19g2000prg.googlegroups.com...
> |I would like to use the following recipe to transpose a list of lists
> | with different lengths.http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/410687
> |
> | Here is an example of what I would like to do:
> |
> | Python 2.5.1 (r251:54863, May 18 2007, 16:56:43)
> | [GCC 3.4.4 (cygming special, gdc 0.12, using dmd 0.125)] on cygwin
> | Type "help", "copyright", "credits" or "license" for more information.
> | >>> a = [[1,2,3],[4,5,6,7],[8,9]]
> | >>> print map(lambda *row: list(row), *a)
> | [[1, 4, 8], [2, 5, 9], [3, 6, None], [None, 7, None]]
> | >>>
> |
> | However, in the Python 3000 FAQ (http://www.artima.com/weblogs/
> | viewpost.jsp?thread=211200), Guido says not to use map with lambda
> | because a list comprehension is clearer and faster.
> |
> | How can I rewrite the above recipe using a list comprehension instead?
>
> Here is the sort of thing Guido is talking about:
>
> >>> a = [[1,2,3],[4,5,6,7],[8,9]]
> >>> [list(r) for r in zip(*a)]
>
> [[1, 4, 8], [2, 5, 9]]
>
> Except in this case, zip does not pad, while map does, so the list comp
> form has to be
>
> >>> [list(r) for r in map(None, *a)]
>
> [[1, 4, 8], [2, 5, 9], [3, 6, None], [None, 7, None]]
>
> but at this point, it is about as easy to put the lambda in place of None.
>
> But when one maps one sequence or equal-length sequences, Guido's point
> applies.
>
> tjr

Thank you very much.