How to get the least common element in a list?

Question

To find the most common, I know I can use something like this:

most_common = collections.Counter(list).most_common(to_find)

However, I can't seem to find anything comparable, for finding the least common element.

Could I please get recommendations on how to do.

Answer 1

Borrowing the source of collections.Counter.most_common and inverting as appropriate:

from operator import itemgetter
import heapq
import collections
def least_common_values(array, to_find=None):
    counter = collections.Counter(array)
    if to_find is None:
        return sorted(counter.items(), key=itemgetter(1), reverse=False)
    return heapq.nsmallest(to_find, counter.items(), key=itemgetter(1))

>>> data = [1,1,2,2,2,2,3,3,3,3,3,3,3,4,4,4,4,4,4,4]
>>> least_common_values(data, 2)
[(1, 2), (2, 4)]
>>> least_common_values([1,1,2,3,3])
[(2, 1), (1, 2), (3, 2)]
>>>