Sort list of lists first by year, then by month

Question

I have a list of lists that looks something like this:

dateList = [['2014', '4'], ['2015', '6'], ['2017', '6'], ['2016', '2'], ['2016', '3'], ['2017', '9'], ['2016', '6'], ['2017', '3'], ['2014', '8'], ['2014', '10'], ['2017', '10'], ['2014', '9'], ['2014', '3'], ['2015', '11'], ['2015', '2']]

I have been trying to sort it by year first, and then by month. I've been successful in sorting it by year using

sortedList = sorted(dateList, key = lambda x: int(x[0]))

However, I can't figure out a way to sort it by month afterwards while keeping the years in ascending order.

@jonrsharpe: Without the `int`, 10 would come before 9 for the months. — fuglede, Dec 17 '17 at 09:39
@fuglede good point, but then *only* the months need to be converted, the OP's current code doesn't gain any benefit. — jonrsharpe, Dec 17 '17 at 09:40

Anton vBR · Accepted Answer · 2017-12-17T10:03:12.923

5

Pass a tuple:

sorted(dateList, key = lambda x: (int(x[0]),int(x[1])))

But this looks better:

sorted(dateList, key = lambda x: tuple(map(int,x)))

And another alternative could be to use:

import datetime 
sorted(dateList, key = lambda x: datetime.datetime.strptime(''.join(x),'%Y%m'))

edited Dec 17 '17 at 10:03

answered Dec 17 '17 at 09:37

Anton vBR

18,287
5
40
46

1

Thanks, I used the first one as it is more in line with my code and seems to be more pythonic. – Robert C Dec 17 '17 at 09:51
1

You're welcome. The 2nd one is more pythonic. – Anton vBR Dec 17 '17 at 09:58

score 3 · Answer 2 · answered Dec 17 '17 at 09:40

It seems like you're in need of a natural sorting of your data. You can use the natsort package to do this, without having to pass a key at all -

import natsort

natsort.natsorted(dateList)

[['2014', '3'],
 ['2014', '4'],
 ['2014', '8'],
 ['2014', '9'],
 ['2014', '10'],
 ['2015', '2'],
 ['2015', '6'],
 ['2015', '11'],
 ['2016', '2'],
 ['2016', '3'],
 ['2016', '6'],
 ['2017', '3'],
 ['2017', '6'],
 ['2017', '9'],
 ['2017', '10']]

Sort list of lists first by year, then by month

2 Answers2