Python list sort in descending order

Question

How can I sort this list in descending order?

timestamps = [
    "2010-04-20 10:07:30",
    "2010-04-20 10:07:38",
    "2010-04-20 10:07:52",
    "2010-04-20 10:08:22",
    "2010-04-20 10:08:22",
    "2010-04-20 10:09:46",
    "2010-04-20 10:10:37",
    "2010-04-20 10:10:58",
    "2010-04-20 10:11:50",
    "2010-04-20 10:12:13",
    "2010-04-20 10:12:13",
    "2010-04-20 10:25:38"
]

score 560 · Answer 1 · edited Nov 05 '21 at 22:05

560

This will give you a sorted version of the array.

sorted(timestamps, reverse=True)

If you want to sort in-place:

timestamps.sort(reverse=True)

Check the docs at Sorting HOW TO

edited Nov 05 '21 at 22:05

Ricardo

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answered Nov 15 '10 at 10:42

Marcelo Cantos

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`reverse` was added in 2.4. But note that `sort()` is stable, so the two bits of code given won't necessarily give the same result. – Ignacio Vazquez-Abrams Nov 15 '10 at 10:49
6

@Rajeev - don't forget you can sort dates only if they are written in this way (YYYY-MM-DD HH:MM:SS), where alphabetically is the same like chronologically. 'DD.MM.YYYY' would be a good example, where you would need more than just `sort(reverse=True)`. – eumiro Nov 15 '10 at 11:28

score 426 · Accepted Answer · edited Dec 04 '20 at 19:07

426

In one line, using a lambda:

timestamps.sort(key=lambda x: time.strptime(x, '%Y-%m-%d %H:%M:%S')[0:6], reverse=True)

Passing a function to list.sort:

def foo(x):
    return time.strptime(x, '%Y-%m-%d %H:%M:%S')[0:6]

timestamps.sort(key=foo, reverse=True)

edited Dec 04 '20 at 19:07

theonlygusti

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answered Nov 15 '10 at 10:42

Ignacio Vazquez-Abrams

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The conversion to a time tuple is unnecessary. – Marcelo Cantos Nov 15 '10 at 10:48
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@Marcelo: Only by coincidence. – Ignacio Vazquez-Abrams Nov 15 '10 at 10:50
23

@IgnacioVazquez-Abrams no, not by coincidence. ISO 8601 is expressly designed so that alphabetical order coincides with chronological order. – jwg Oct 08 '15 at 08:32

score 66 · Answer 3 · edited Dec 04 '20 at 19:08

66

You can simply do this:

timestamps.sort(reverse=True)

edited Dec 04 '20 at 19:08

theonlygusti

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answered Nov 15 '10 at 10:44

Wolph

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score 13 · Answer 4 · edited Dec 04 '20 at 19:08

13

you simple type:

timestamps.sort()
timestamps=timestamps[::-1]

edited Dec 04 '20 at 19:08

theonlygusti

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answered Dec 18 '16 at 16:18

mostafa elmadany

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This is a strange answer because you do the sorting in-place but then the reversing out-of-place. If there is another variable aliasing the original list, its value afterwards will not have the elements in their original order, nor in descending order; the alias will point at a list sorted in *ascending* order. That could be rather surprising, and a source of subtle bugs. – kaya3 Nov 10 '19 at 16:44

score 11 · Answer 5 · edited Dec 04 '20 at 19:08

Since your list is already in ascending order, we can simply reverse the list.

>>> timestamps.reverse()
>>> timestamps
['2010-04-20 10:25:38', 
'2010-04-20 10:12:13', 
'2010-04-20 10:12:13', 
'2010-04-20 10:11:50', 
'2010-04-20 10:10:58', 
'2010-04-20 10:10:37', 
'2010-04-20 10:09:46', 
'2010-04-20 10:08:22',
'2010-04-20 10:08:22', 
'2010-04-20 10:07:52', 
'2010-04-20 10:07:38', 
'2010-04-20 10:07:30']

score 5 · Answer 6 · edited Dec 04 '20 at 19:09

5

Here is another way


timestamps.sort()
timestamps.reverse()
print(timestamps)

edited Dec 04 '20 at 19:09

theonlygusti

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answered Mar 17 '20 at 14:20

hamdan

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Never knew about `reverse()`, super useful – Hamman Samuel Aug 10 '21 at 01:24

score 1 · Answer 7 · answered Mar 16 '23 at 18:11

Especially if the data is numeric, negation can be used to sort in descending order. This is especially useful if you need to pass a sorting key anyway. For example, if the data was as follows:

data = ['9', '10', '3', '4.5']
sorted(data, reverse=True)                      # doesn't sort correctly
sorted(data, key=lambda x: -float(x))           # sorts correctly
#                          ^ negate here

# that said, passing a key along with reverse=True also work
sorted(data, key=float, reverse=True)           # ['10', '9', '4.5', '3']

For an example with datetime, that would look like as follows:

from datetime import datetime
ts = ["04/20/2010 10:07:30", "12/01/2009 10:07:52", "01/13/2020 10:08:22", "12/01/2009 12:07:52"]
ts.sort(key=lambda x: -datetime.strptime(x, '%m/%d/%Y %H:%M:%S').timestamp())
#                                                               ^^^^ convert to a number here
ts
# ['01/13/2020 10:08:22', '04/20/2010 10:07:30', '12/01/2009 12:07:52', '12/01/2009 10:07:52']

Python list sort in descending order

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