按频率排序，而不是按字母顺序排列（在Python中）

Question

I was looking at an example in the Python library where the total number of occurrences of a word in a list are ordered by their frequency count in a dictionary:

cnt = Counter()
for word in ['red', 'blue', 'red', 'green', 'blue', 'blue']:
    cnt[word] += 1

The output for cnt where the elements are listed in order of their frequency count:

Counter({'blue': 3, 'red': 2, 'green': 1})

I want to make a set where the numerical frequencies aren't included, but the order the elements are listed is retained:

{'blue', 'red', 'green'}

However in my attempt to achieve this:

set(word for word, count in cnt.most_common(3))

I instead receive a set where the elements are ordered alphabetically instead of frequency count:

{'blue', 'green', 'red'}

Is it possible to order the set according to the frequency count?

Answer 1

Sets are a unordered collection of unique elements, as such ordering them has no meaning whatsoever.

You probably want a list, which is ordered. The following uses a list comprehension to construct a list of the keys.

We can iterate over Counter.most_common() which returns a sequence of (key, value) tuples in order.

from collections import Counter

c = Counter({'blue': 3, 'red': 2, 'green': 1})

keys = [key for key, val in c.most_common()]

print(keys)
# ['blue', 'red', 'green']