Python itertools - Combining groupby and recipe tools' grouper -

say have following data:

data = [['john', 1], ['ada', 2], ['ada', 3], ['paul', 4],         ['paul', 5], ['paul', 6], ['kat', 7], ['kat', 8]] 

i can group entries person groupby:

in [37]:  itertools import groupby, izip_longest operator import itemgetter  name, g in groupby(data, key=itemgetter(0)):     print name, list(g)  john [['john', 1]] ada [['ada', 2], ['ada', 3]] paul [['paul', 4], ['paul', 5], ['paul', 6]] kat [['kat', 7], ['kat', 8]] 

i can group every 2 entries use recipe tools' grouper. copy/paste reference:

in [38]:  def grouper(iterable, n, fillvalue=none):     "collect data fixed-length chunks or blocks"     # grouper('abcdefg', 3, 'x') --> abc def gxx     args = [iter(iterable)] * n     return izip_longest(fillvalue=fillvalue, *args)  g in grouper(data, 2):     print g (['john', 1], ['ada', 2]) (['ada', 3], ['paul', 4]) (['paul', 5], ['paul', 6]) (['kat', 7], ['kat', 8]) 

but now, want iterate on data such first element contains john , ada's data, , second element contains paul , kat's data. in other words, combine groupby , grouper this:

in [39]:  person_iterator = groupby(data, key=itemgetter(0)) group_iterator in grouper(person_iterator, 2):     print [(keyvalue[0], list(keyvalue[1])) keyvalue in group_iterator] 

but output not expected:

[('john', []), ('ada', [['ada', 2], ['ada', 3]])] [('paul', []), ('kat', [['kat', 7], ['kat', 8]])] 

why there empty lists john , paul? how fix it?

a iterator yielded (group_iterator[1]) itertools.groupby exhausted when next iterator yielded.

you need convert iterators sequences before passing grouper prevent that:

person_iterator = ((key, list(grp)) key, grp in groupby(data, key=itemgetter(0))) group_iterator in grouper(person_iterator, 2):     print [(key, value) key, value in group_iterator] 

output:

[('john', [['john', 1]]), ('ada', [['ada', 2], ['ada', 3]])] [('paul', [['paul', 4], ['paul', 5], ['paul', 6]]), ('kat', [['kat', 7], ['kat', 8]])]