How do I sort a list of datetime or date objects?

Question

How do I sort a list of date and/or datetime objects? The accepted answer here isn't working for me:

from datetime import datetime,date,timedelta


a=[date.today(), date.today() + timedelta(days=1), date.today() - timedelta(days=1)]
print a # prints '[datetime.date(2013, 1, 22), datetime.date(2013, 1, 23), datetime.date(2013, 1, 21)]'
a = a.sort()
print a # prints 'None'....what???

Solution

You're getting None because list.sort() it operates in-place, meaning that it doesn't return anything, but modifies the list itself. You only need to call a.sort() without assigning it to a again.

There is a built in function sorted(), which returns a sorted version of the list - a = sorted(a) will do what you want as well.

Comments

[radtek]

which way is best? do they do same things under the covers?

[flyman]

@radtek list.sort() changes the object from which it is invoked (which is always a list). sorted() works on any iterable, not only list. list.sort() might be a little faster if you have a very large list because sorted() creates a new list and needs to copy the elements to it.