Sort by elements of a list of tuples

Question

I have a list of tuples or a tuple of tuples and I want to swap the tuples like in the next example:

>>> swap([('abc', 'bcd'), ('efg', 'zza')])
[('efg', 'zza'), ('abc', 'bcd')]

I want to swap the tuples from the list based on the third character of the second element of every tuple.

Are you trying to reverse the list, or swap the values? In your real list, do both tuples start with `'abc'`? — Adam Smith, Oct 13 '17 at 22:05
Or: better question. If your test data was `[(1, 2), (3, 4)]` would your intended result be `[(3, 4), (1, 2)]` or `[(1, 3), (2, 4)]` (or `[(1, 4), (3, 2)]`)? — Adam Smith, Oct 13 '17 at 22:06
Well my problem is that i have more than 2 elements in my main list, and i need to swap them all... — Apetrei Alin, Oct 13 '17 at 22:07
@AdamSmith yes both of them start with 'abc', it's about swaping them based on the 3rd character of the 2nd element in the tuple — Apetrei Alin, Oct 13 '17 at 22:08
@ApetreiAlin your intended functionality is unclear. On what basis is the 3rd character of the 2nd element of the tuple compared? You'll need to be *much* more specific. Give us some real data. — Adam Smith, Oct 13 '17 at 22:10
Well in this example we compare 'd' from the second element 'bcd' with 'a' from 'zza', i hope u get it now. I have *args parameters, so something like args[0][1][2] compare with args[1][1][2] — Apetrei Alin, Oct 13 '17 at 22:12
@ApetreiAlin so what you really want is to sort the list by the 3rd character of the second element of each tuple? — Adam Smith, Oct 13 '17 at 22:13
(sorry to duplicate close voters, but it became obvious that the question he was asking was not the *question he asked*. The latter was exactly the dupe target. The former is a different dupe, I'm sure, but I don't have it handy.) — Adam Smith, Oct 13 '17 at 22:19
@MosesKoledoye courtesy ping. I asked about this [on meta](https://meta.stackoverflow.com/questions/357824/should-i-use-my-dupehammer-to-open-an-xy-problem) — Adam Smith, Oct 13 '17 at 22:43

Adam Smith · Accepted Answer · 2017-10-13T22:21:11.447

Hoo boy, this was a big XY problem. You want to sort by the third character of the second element of each tuple. Use the sorted built-in with its kwarg key.

lst = [('abc', 'bcd'), ('abc', 'zza')]
result = sorted(lst, key=lambda t: t[1][2])

key here accepts a function that is given each element in turn, and does something to it that is compared. In this case it returns the second element's third element (t[1][2]). Each element of the outer list is a tuple. Each tuple's second element exists in index 1, each second element's third character exists in index 2.

Sort by elements of a list of tuples

1 Answers1