How to make another list of the duplicate entries in existing list using python?

Question

I had a list like:

l = [[(3,4)], [(3,7)], [(3,8)]]

I used the chain() function to flat the list, now I have a list like:

l2 = [3,4,3,7,3,8]

I want to separate the duplicate items into another list:

l3 = [3,3,3]
l4 = [4,7,8]

I used the set() function, but it destroyed the duplicate items and resulted in:

l3 = [4,7,8]

but I want to obtain both of them separately

Answer 1

You'd have to do some kind of count. Using the collections.Counter() class would make that easy:

from collections import Counter
counts = Counter(main_list)

duplicate_list, unique_list = [], []
for entry in main_list:
    if counts[entry] > 1:
        duplicate_list.append(entry)
    else:
        unique_list.append(entry)

counts is a multi-set or bag; a mapping of entry to it's count in main_list. The above example preserves the ordering of main_list.

Answer 2

extention to @Martijn Pieters answer, you can use ifilter to filter out those that match the criteria in the lambda expression(first argument in ifilter):

from collections import Counter
from itertools import ifilter

counts = Counter(l2)    

l3 = list(ifilter(lambda x: counts[x] > 1, l2))
l4 = list(ifilter(lambda x: counts[x] == 1, l2))