Delete unique elements from a list

Question

I faced some problem with solving the next problem:

We have a list of elements (integers), and we should return a list consisting of only the non-unique elements in

Answer 1

You can implement a OrderedCounter, eg:

from collections import OrderedDict, Counter

class OrderedCounter(Counter, OrderedDict): 
    pass

data = [1, 3, 1, 2, 3, 5, 8, 1, 5, 2]

duplicates = [k for k, v in OrderedCounter(data).items() if v > 1]
# [1, 3, 2, 5]

So you count the occurrence of each value, then filter on if it has a frequency of more than one. Inheriting from OrderedDict means the order of the original elements is preserved.

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Going by comments, you want all duplicated elements reserved, so you can pre-build a set of the duplicate entries, then re-iterate your original list, eg:

from collections import Counter

data = [1, 3, 1, 2, 3, 5, 8, 1, 5, 2]
duplicates = {k for k, v in Counter(data).items() if v > 1}
result = [el for el in data if el in duplicates]
# [1, 3, 1, 2, 3, 5, 1, 5, 2]