enumerate
>>> def _treatment(pos,element): return '%d:%s' % (pos, element) >>> [_treatment(i, el) for i, el in enumerate([chr(x) for x in xrange(65,91)])] ['0:A', '1:B', '2:C', '3:D', '4:E', '5:F', '6:G', '7:H', '8:I', '9:J', '10:K', '11:L', '12:M', '13:N', '14:O', '15:P', '16:Q', '17:R', '18:S', '19:T', '20:U', '21:V', '22:W', '23:X', '24:Y', '25:Z']