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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?

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2 Answers 2

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

Actually I don't think you want that reverse=True.
Yeah, I already removed it; had a slight brainfart about "oldest".
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.
I'll raise it as a new question
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.
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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

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2 Comments

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.
@J.F. Sebastian: agreed, I just quoted the wiki:) I'll edit the answer

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