How to sort a Python list descending based on item count, but return a list with sorted items only, and not a count also?

Question

I am generating some large lists in python, and I need to figure out the absolute fastest way to return a sort list from something like this:

myList = ['a','b','c','a','c','a']

and return a list of it in descending order, based on the count of the items, so it looks like this.

sortedList = ['a','c','b']

I have been using Counter().most_common(), but this returns a tuple in descending order with the item, and the number of times in appears in the list. I really just need a tuple or list in descending order based off count, with just the items, and not the count amounts. Any ideas?

Edit: So would doing something like this be faster?

myList = ['a','b','c','a','c','a']
count = Counter(myList).most_common()
res = [k for k,v in count]

Just list-comp over the most_common to only select the key? — Jon Clements
– Jon Clements, Commented May 21, 2014 at 17:17
sorted(counter_obj, key=counter_obj.__getitem__, reverse=True) — Ashwini Chaudhary
– Ashwini Chaudhary, Commented May 21, 2014 at 17:17

Jon Clements · Accepted Answer · 2014-05-21 17:20:42Z

5

from collections import Counter

myList = ['a','b','c','a','c','a']
res = [k for k,v in Counter(myList).most_common()]
# ['a', 'c', 'b']

answered May 21, 2014 at 17:20

Jon Clements

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11 Comments

Ashwini Chaudhary Over a year ago

Not the fastest way though, no need to introduce a Python for-loop.

Jon Clements Over a year ago

@200OK Since .most_common() is sorting anyway... no need to pre-generate a Counter, then sort using its values using a look up either...

roippi Over a year ago

@200OK a comprehension loops at C speed so I'm not sure what your point is. The fact that comprehension grammar includes the word "for" doesn't means it performs like an actual for loop.

DSM Over a year ago

On a test I just ran comparing 1M choices of 'a','b','c', I got exactly 139 ms for each, so whatever minor performance differences there are probably wash out at large size anyway.

Ashwini Chaudhary Over a year ago

@JonClements I thought OP wanted to use the Counter later as well, if that's not the case then this is fastest solution I guess.

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