Sort list by frequency

Question

Is there any way in Python, wherein I can sort a list by its frequency?

For example,

[1,2,3,4,3,3,3,6,7,1,1,9,3,2]

the above list

Answer 1

In case you want to use a double comparator.

For example: Sort the list by frequency in descending order and in case of a clash the smaller one comes first.

import collections 

def frequency_sort(a):
    f = collections.Counter(a)
    a.sort(key = lambda x:(-f[x], x))
    return a