itertools: problem with nested groupby, list()

[Jon Clements]

On 4 May, 11:10, Nico Schlömer <nico.schloe... at gmail.com> wrote:
> Hi,
>
> I ran into a bit of an unexpected issue here with itertools, and I
> need to say that I discovered itertools only recently, so maybe my way
> of approaching the problem is "not what I want to do".
>
> Anyway, the problem is the following:
> I have a list of dictionaries, something like
>
> [ { "a": 1, "b": 1, "c": 3 },
>   { "a": 1, "b": 1, "c": 4 },
>   ...
> ]
>
> and I'd like to iterate through all items with, e.g., "a":1. What I do
> is sort and then groupby,
>
> my_list.sort( key=operator.itemgetter('a') )
> my_list_grouped = itertools.groupby( my_list, operator.itemgetter('a') )
>
> and then just very simply iterate over my_list_grouped,
>
> for my_item in my_list_grouped:
>     # do something with my_item[0], my_item[1]
>
> Now, inside this loop I'd like to again iterate over all items with
> the same 'b'-value -- no problem, just do the above inside the loop:
>
> for my_item in my_list_grouped:
>         # group by keyword "b"
>         my_list2 = list( my_item[1] )
>         my_list2.sort( key=operator.itemgetter('b') )
>         my_list_grouped = itertools.groupby( my_list2,
> operator.itemgetter('b') )
>         for e in my_list_grouped:
>             # do something with e[0], e[1]
>
> That seems to work all right.
>
> Now, the problem occurs when this all is wrapped into an outer loop, such as
>
> for k in [ 'first pass', 'second pass' ]:
>     for my_item in my_list_grouped:
>     # bla, the above
>
> To be able to iterate more than once through my_list_grouped, I have
> to convert it into a list first, so outside all loops, I go like
>
> my_list.sort( key=operator.itemgetter('a') )
> my_list_grouped = itertools.groupby( my_list, operator.itemgetter('a') )
> my_list_grouped = list( my_list_grouped )
>
> This, however, makes it impossible to do the inner sort and
> groupby-operation; you just get the very first element, and that's it.
>
> An example file is attached.
>
> Hints, anyone?
>
> Cheers,
> Nico

Does this example help at all?

my_list.sort( key=itemgetter('a','b','c') )
for a, a_iter in groupby(my_list, itemgetter('a')):
    print 'New A', a
    for b, b_iter in groupby(a_iter, itemgetter('b')):
        print '\t', 'New B', b
        for c, c_iter in groupby(b_iter, itemgetter('c')):
            print '\t'*2, 'New C', c
            for c_data in c_iter:
                print '\t'*3, a, b, c, c_data
            print '\t'*2, 'End C', c
        print '\t', 'End B', b
    print 'End A', a

Jon.