You can use the max() function to find the most common element in a given list.
Here is an example:
>>> aa = [1, 2, 3, 4, 4, 4, 5, 5]>>> max(aa, key=aa.count)4
>>> aa = [1, 2, 3, 4, 4, 4, 5, 5]
>>> max(aa, key=aa.count)
4