Python: fast way to create a list of reps

Question

I have a list: list = [A, B, C, D, A, A, A, B, E, C, C, E, D, A, B] I want to count appearance of every element, so I can use function count . But I want to create a table which contains a tuple (element, number_of_reps) , it is: [(A, 5), (B, 3), (C, 3), (D, 2), (E, 2)].

I wanted to make it as:

newlist = []
for i in list:
 rep = list.count(i)
 tup = (i, rep)
 if tup not in newlist:
   newlist.append(tup)

But can I do it better: faster or using any built in function?

Answer 1

Use collections.Counter . It has a most_common method which does exactly what you want...

>>> import collections
>>> A, B, C, D, E = 'ABCDE'
>>> lst = [A, B, C, D, A, A, A, B, E, C, C, E, D, A, B]
>>> collections.Counter(lst).most_common()
[('A', 5), ('C', 3), ('B', 3), ('E', 2), ('D', 2)]

The Counter is a dictionary which holds a mapping of the keys to the number of times that key was seen in the input iterable.

>>> collections.Counter(lst)
Counter({'A': 5, 'C': 3, 'B': 3, 'E': 2, 'D': 2})

most_common is just a simple method to get a handle on the N most common elements in the input -- with no arguments, it just sorts the items by the number of counts.

Answer 2

Use collections.Counter . The interface is similar to that of a dictionary, and you can call most_common to give exactly the output you want:

>>> import collections
>>> some_list = ["A", "B", "C", "D", "A", "A", "A", "B", "E", "C", "C", "E", "D", "A", "B"]
>>> counter = collections.Counter(some_list)
>>> counter.most_common()
[('A', 5), ('C', 3), ('B', 3), ('E', 2), ('D', 2)]