Count the frequency of an item in a python list

To count frequency in list you can do as shown : 

To count the number of appearances:  from collections import defaultdict

appearances = defaultdict(int)

for curr in a:

    appearances[curr] += 1

To remove duplicates:

a = set(a)

Example:

import collections

a = [1,1,1,1,2,2,2,2,3,3,4,5,5]

counter=collections.Counter(a)

print(counter)

# Counter({1: 4, 2: 4, 3: 2, 5: 2, 4: 1})

print(counter.values())

# [4, 4, 2, 1, 2]

print(counter.keys())

# [1, 2, 3, 4, 5]

print(counter.most_common(3))

# [(1, 4), (2, 4), (3, 2)]