how to sort a list of list of tuples in python?

Question

how do you sort a list of list of tuples in python

for example: Note that the last elements of the tuples are same

a  = [

[(1, 5, 6), (2, 2, 6), (3, 6, 6)],
[(6, 1, 3), (5, 7, 3), (4, 1, 3)],
[(5, 7, 2), (7, 5, 2), (6, 3, 2)],
[(9, 1, 7), (1, 5, 7), (2, 6, 7)]

]

I want to sort using the last element of the tuple as the key. So the expected result is

[

[(5, 7, 2), (7, 5, 2), (6, 3, 2)],
[(6, 1, 3), (5, 7, 3), (4, 1, 3)],
[(1, 5, 6), (2, 2, 6), (3, 6, 6)],
[(9, 1, 7), (1, 5, 7), (2, 6, 7)]

]

Answer 1

In [27]: a  = [
   ....:
   ....: [(1, 5, 6), (2, 2, 6), (3, 6, 6)],
   ....: [(6, 1, 3), (5, 7, 3), (4, 1, 3)],
   ....: [(5, 7, 2), (7, 5, 2), (6, 3, 2)],
   ....: [(9, 1, 7), (1, 5, 7), (2, 6, 7)]
   ....:
   ....: ]
                               # any tuple (first will do), last element
In [28]: a.sort(key=lambda l: l[0][-1])

In [29]: a
Out[29]:
[[(5, 7, 2), (7, 5, 2), (6, 3, 2)],
 [(6, 1, 3), (5, 7, 3), (4, 1, 3)],
 [(1, 5, 6), (2, 2, 6), (3, 6, 6)],
 [(9, 1, 7), (1, 5, 7), (2, 6, 7)]]

Answer 2

Use the key argument to sort and a lambda function

a.sort(key=lambda x: x[-1][-1]). # sorts in-place
sorted(a, key=lambda x: x[-1][-1]). # new sorted list

Edit : change the first index depending on what tuple you want to look at for the comparison. Ie, use x[0][-1] to compare based on the first tuple