I have a tuple in the following format:
(639283, 298290710, 1385)
(639283, 298290712, 1389)
(639283, 298290715, 1395)
(745310, 470212995, 2061)
(745310, 47021382
You can use itertools.groupby to group the tuples based on the first item, and then iterate over those groups in a dict-comprehension to get the desired result.
>>> from operator import itemgetter
>>> from pprint import pprint
>>> from itertools import groupby
>>> d = {k: dict(x[1:] for x in g) for k, g in groupby(data, key=itemgetter(0))}
>>> pprint(d)
{639283: {298290710: 1385, 298290712: 1389, 298290715: 1395},
745310: {470212995: 2061,
470213821: 3713,
470215360: 6791,
470215361: 6793,
470215363: 6797},
911045: {374330803: 4905,
374330804: 4907,
374330807: 4913,
374330808: 4915,
374330809: 4917}}
Where data is your list of tuples or tuple of tuples.