Sorting tuples by element value in Python

Question

I need to sort a list of tuples in Python by a specific tuple element, let's say it's the second element in this case. I tried

sorted(tupList, key = lambda tup: tup[1])

I have also tried

sorted(tupList, key = itemgetter(1))
'''i imported itemgetter, attrgetter, methodcaller from operator'''

but the list was returned the same both times. I checked

Reminder: calling sorted on a list doesn't modify the original list. If you're doing sorted(seq); print(seq);, then you're not going to see any changes. Make sure you're doing new_thing = sorted(seq); print(new_thing) or similar. — Kevin
– Kevin, Commented Apr 15, 2015 at 17:09
o_O Your code seems fine to me. Show input and expected output please? — Shashank
– Shashank, Commented Apr 15, 2015 at 17:10
As is stated by @Kevin, sorted does not mutate a list in place, but returns a new version of it. If you want to mutate in place, you should use tupList.sort(key=lambda t: t[1]) — Christian Witts
– Christian Witts, Commented Apr 15, 2015 at 17:14
@Kevin you are right. I switched from list.sort() to sorted and forgot I have to save the result somewhere. Please make an answer so I can accept. — user137717
– user137717, Commented Apr 15, 2015 at 17:16

Kevin · Accepted Answer · 2015-04-15 17:21:49Z

1

I'm guessing you're calling sorted but not assigning the result anywhere. Something like:

tupList = [(2,16), (4, 42), (3, 23)]
sorted(tupList, key = lambda tup: tup[1])
print(tupList)

sorted creates a new sorted list, rather than modifying the original one. Try:

tupList = [(2,16), (4, 42), (3, 23)]
tupList = sorted(tupList, key = lambda tup: tup[1])
print(tupList)

Or:

tupList = [(2,16), (4, 42), (3, 23)]
tupList.sort(key = lambda tup: tup[1])
print(tupList)

answered Apr 15, 2015 at 17:21

Kevin

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