Simple list problem that's defeating me!

[Sion Arrowsmith]

Mark Lawrence  <breamoreboy at yahoo.co.uk> wrote:
>On 22/06/2010 15:06, Neil Webster wrote:
>> I have a list of lists such as [[a,2,3,4],[b,10,11,12], [a,2,3,4]].  I
>> need to combine the two lists that have the same first character in
>> this example 'a'.  In reality there are 656 lists within the list.
>> [ ... ]
>My simplistic approach.
>
>Sort the list (this happens in place).
>Use the itertools groupby function to place everything together, see
>http://docs.python.org/library/itertools.html?highlight=groupby#itertools.groupby
>Combine the lists in the groups.

I suspect the following is a more efficient way of acheiving the grouping:

d = collections.defaultdict(list)
for a in L:
    d[a[0]].append(a[1:])

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