python - Sort list of strings by length and alphabetically

Question

I need to sort a list of words based on two criteria given. I need to return a list with the same words in order of length (longest to shortest) and the second sort criteria should be alphabetical.

Example list :

l = ['aa','aaa','aaaa','b','bb','z','ccc']

Desired output:

['aaaa', 'aaa', 'ccc', 'aa', 'bb', 'b', 'z']

Answer 1

You only need one call to sort, because Python automatically sorts tuples lexicographically. That is, if you ask Python to compare two tuples it will order them by their first element, except if those compare equal in which case it will order them by their second element, except if those compare equal in which case...

You want to sort the list of elements by minus their length and then alphabetically, so you want the key of a string s to be the tuple (-len(s), s). Hence:

>>> l = ['aa','aaa','aaaa','b','bb','z','ccc']
>>> sort_key = lambda s: (-len(s), s)
>>> l.sort(key=sort_key)
>>> l
['aaaa', 'aaa', 'ccc', 'aa', 'bb', 'b', 'z']