Sort Python list of objects by date

Question

I have a Python list called results. Each result in the results list has a person object, and each person object has a birthdate (result.person.birthdate). The birthdate is a datetime object.

I would like to order the list by birthdate with the oldest first. What is the most Pythonic way to do this?

score 81 · Accepted Answer · answered Feb 20 '11 at 07:28

81

results.sort(key=lambda r: r.person.birthdate)

answered Feb 20 '11 at 07:28

Amber

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Actually I don't think you want that `reverse=True`. – rlibby Feb 20 '11 at 07:31
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Yeah, I already removed it; had a slight brainfart about "oldest". – Amber Feb 20 '11 at 07:32
Just noticed some of the objects have person object set to None. I still want these results in the list. How can I do this while still sorting the others? Thanks. – shane Feb 20 '11 at 07:48
I'll raise it as a new question – shane Feb 20 '11 at 08:06
@shane: `results.sort(key=lambda r: getattr(r.person, 'birthday', datetime.max))` – jfs Oct 12 '14 at 16:59
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Watch out: list.sort sorts in-place instead of returning the sorted list. Use `sorted(results, key=...)` if you don't want to mutate the original list. – Paulo Scardine Oct 20 '14 at 13:12

yentsun · Answer 2 · 2014-10-21T05:29:39.757

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Totally agree with Amber, but there is another way of sorting by attribute (from the wiki: https://wiki.python.org/moin/HowTo/Sorting):

from operator import attrgetter
sorted_list = sorted(results, key=attrgetter('person.birthdate'))

This method can actually be even faster than sorting with lambda

edited Oct 21 '14 at 05:29

answered Oct 10 '14 at 07:41

yentsun

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`attrgetter()` is convenient if `'person.birdate' is passed as a variable. Otherwise the `lambda` is more flexible (e.g., what if `person` is None sometimes) and the time difference should be negligible in most cases. – jfs Oct 12 '14 at 16:57
@J.F. Sebastian: agreed, I just quoted the wiki:) I'll edit the answer – yentsun Oct 20 '14 at 13:04

Sort Python list of objects by date

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