How to get unique values with respective occurrence count from a list in Python?

Question

I have a list which has repeating items and I want a list of the unique items with their frequency.

For example, I have [\'a\', \'a\', \'b\', \'b\', \'b\']

Answer 1

If you are willing to use a 3rd party library, NumPy offers a convenient solution. This is particularly efficient if your list contains only numeric data.

import numpy as np

L = ['a', 'a', 'b', 'b', 'b']

res = list(zip(*np.unique(L, return_counts=True)))

# [('a', 2), ('b', 3)]

To understand the syntax, note np.unique here returns a tuple of unique values and counts:

uniq, counts = np.unique(L, return_counts=True)

print(uniq)    # ['a' 'b']
print(counts)  # [2 3]

See also: What are the advantages of NumPy over regular Python lists?