How to use a custom predicate sort with Python

Question

I have a list of lists like [[1,2,"s"],[1,5,"e"],...] where the 3rd value is always either s or e. How can I call sort() such that the list is sorted based on:

the first index
if the first index is same, s comes first.

Thanks

niemmi · Accepted Answer · 2016-05-27 15:48:56Z

3

You can pass custom key function to list.sort that is used to generate the comparison key for the items:

>>> l = [[1, 5, 'e'], [1, 2, 's'], [0, 4, 'e']]
>>> l.sort(key=lambda x: (x[0], -ord(x[2])))
>>> l
[[0, 4, 'e'], [1, 2, 's'], [1, 5, 'e']]

edited May 27, 2016 at 15:48

answered May 27, 2016 at 15:42

niemmi

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Comments

Jared Goguen · Accepted Answer · 2016-05-27 15:42:44Z

1

Write a custom key function.

def sort_key(list_):
    if list_[2] == 's':
        return (list_[0], 0)
    return (list_[0], 1)

test_list = [[1, 2, 's'], [1, 5, 'e'], [2, 4, 'e'], [2, 3, 's']]
test_list.sort(key=sort_key)
# [[1, 2, 's'], [1, 5, 'e'], [2, 3, 's'], [2, 4, 'e']]

answered May 27, 2016 at 15:42

Jared Goguen

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