Sort list of strings by length and alphabetically

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']

Design

The keyword here is "stable sorting algorithm". Think of two stable sorting functions:

• one sorting according to length (maintaining the relative order of entries with equal lengths),
• the other sorting alphabetically.

In which order would you combine them in order to get the desired order?

Implementation

As others have mentioned, the first sorting function can be called like this:

list.sort(key=len, reverse=True)

The second sorting function can be called like this:

list.sort()

This should be enough to write a complete solution.

Result

If you combine the function in the right way, you should get the following:

>>> l = ['aaa', 'fff', 'bbb', 'ddd', 'e', 'cccc']
# ... sorting functions combined in the right way ...
>>> l
['cccc', 'aaa', 'bbb', 'ddd', 'fff', 'e']