If you're using Python 3.4 or above, you can use statistics.mode()
>>> import statistics
>>> L = [1, 2, 45, 55, 5, 4, 4, 4, 4, 4, 4, 5456, 56, 6, 7, 67]
>>> statistics.mode(L)
4
Note that this will throw a statistics.StatisticsError
if the list is empty or if there is not exactly one most common value.