d = [('A', 'B', 1), ('C', 'D', 1),
     ('B', 'D', 2), ('A', 'B', 3),
     ('A', 'D', 3), ('B', 'C', 4),
     ('A', 'C', 5), ('B', 'C', 8)]

d = [('A', 'B', 1), ('C', 'D', 1),
     ('B', 'D', 2), ('A', 'D', 3),
     ('B', 'C', 4), ('A', 'C', 5)]

edge_dict = {}

for x in d:
    key = '%s%s' % (x[0], x[1])
    if not edge_dict.get(key):
        edge_dict[key] = x[2]
    else:
        if edge_dict[key] > x[2]:
            edge_dict[key] = x[2]

final_list = []
for k, v in edge_dict.items():
    t = list(k)
    t.append(v)
    final_list.append(tuple(t))


final_list.sort(key=lambda x: x[2])
print final_list

import itertools
d = [('A', 'B', 1), ('C', 'D', 1),
 ('B', 'D', 2), ('A', 'B', 3),
 ('A', 'D', 3), ('B', 'C', 4),
 ('A', 'C', 5), ('B', 'C', 8)]
new_d = [min(list(b), key=lambda x:x[-1]) for _, b in itertools.groupby(sorted(d, key=lambda x:x[:-1]), key=lambda x:x[:-1])]

[('A', 'B', 1), ('A', 'C', 5), ('A', 'D', 3), ('B', 'C', 4), ('B', 'D', 2), ('C', 'D', 1)]