Python sorting key function supporting tuples and lists

Question

In Python you can for example sort tuples sorted([(2,2),(1,2),(2,1),(1,1)]) and get [(1, 1), (1, 2), (2, 1), (2, 2)].

You can also use custom k

Answer 1

from collections import Iterable
def custom_key(item):
    if isinstance(item, Iterable) and not isinstance(item, basestring):
        return [medals_map.get(ele, ele) for ele in item]
    return medals_map.get(item, item)

Output:

In [2]: assert sorted(['Silver', 'Bronze', 'Gold'], key=custom_key) == ['Gold', 'Silver', 'Bronze']

In [3]: assert sorted([['Silver', 2], ['Silver', 1], ['Gold', 1]], key=custom_key) == [['Gold', 1], ['Silver', 1], ['Silver', 2]]

In [4]: assert sorted([(2, 'Gold'), (2, 'Bronze'), (1, 'Gold')], key=custom_key) == [(1, 'Gold'), (2, 'Gold'), (2, 'Bronze')]

In [5]: assert sorted([('Silver', 'Bronze'), ('Gold', 'Bronze'), ('Silver', 'Gold')], key=custom_key) == [('Gold', 'Bronze'), ('Silver', 'Gold'), ('Silver', 'Bronze')]

Answer 2

You don't need a extra function. You can also do it:

medals_map = {'Gold': 1, 'Silver': 2, 'Bronze': 3 }
List = [('Bronze', 1), ('Gold', 1), ('Gold', 2)]

new_list = sorted(List, key=lambda word: (medals_map[word[0]], word[1]))

print new_list

Output:

[('Gold', 1), ('Gold', 2), ('Bronze', 1)]

Answer 3

The key function can return a tuple:

medals_map = {'Gold': 1, 'Silver': 2, 'Bronze': 3 }

def custom_key(item):
    if isinstance(item, basestring):
       try:
           return medals_map[item]
       except KeyError:
           pass
    return item

print sorted([('Gold', 2), ('Bronze', 1), ('Gold', 1)], key=lambda x: (custom_key(x[0]), x[1]))

Prints [('Gold', 1), ('Gold', 2), ('Bronze', 1)]. Essentially this makes the sorting behave the same as your first example, i.e. sorted([(1, 2), (3, 1), (1, 1)]).

Edit: you can also make a custom key that returns a tuple, which may be prettier.

Answer 4

You could create a mapper function, that takes your key function and returns another function, applying the key function to each element of some iterable.

def mapper(function):
    def inner(values):
        return tuple([function(x) for x in values])
    return inner

Example:

>>>sorted([('Gold', 2), ('Bronze', 1), ('Gold', 1)], key=mapper(custom_key))
[('Gold', 1), ('Gold', 2), ('Bronze', 1)]

Or similar, using functools.partial with map:

>>> sorted([('Gold', 2), ('Bronze', 1), ('Gold', 1)], key=functools.partial(map, custom_key))
[('Gold', 1), ('Gold', 2), ('Bronze', 1)]