Sorting lists in Python (version 3!)

Sorting a 1-dimensional list is easy.

     lst = [4,9,1,3,2]
     lst.sort()

If you need another copy of the list in the new order, you do not want the old list to be modified, use the function (note, not method) sorted()

     lst = [4,9,1,3,2]
     newlst = sorted(lst)

For 2-dimensional lists (lists of lists) the method and function work but they always sort first by the first column, then if needed the second column, and so on.

     lst = [['c', 13], ['b', 10], ['a', 23], ['b', 8]]
     lst.sort()

If you want to sort a 2-dimensional list but NOT using the first column as the order, here is some code that will do it.

from random import randint

# Demonstrates sorting 2-dimensional lists by any column
#  the get_item function should return the element in the desired column

# the only purpose of this function is to return the 2nd element in the
#  1-dimensional list that is the parameter
def get_item(lst):
    return lst[1]


def main():
    # building a list of 8 rows which have 2 random numbers each
    mylst = []
    for i in range(8):
        mylst.append( [randint(10,50), randint(100, 200)] )
        
    print("Original list", mylst)
    print("\nSorted by first column, ascending order", sorted(mylst) ) # sorted by first column
    print("\nSorted by first column, descending order", sorted(mylst, reverse = True))
    print("\n\nSorted by second column, ascending order", sorted(mylst, key=get_item))
    print("\nSorted by second column, descending order", sorted(mylst, key=get_item, reverse = True))
    print("\nOriginal list unchanged", mylst)
main()

Note there is a little function get_item whose only purpose is to return the desired element from one row of the list being sorted.
Note the new syntax, "key=get_item". This is a place where you are providing the NAME of the function, not calling the function. So there are no parentheses after get_item. The sort method or sorted function will call the given function as needed.

Original list [[35, 182], [50, 141], [42, 103], [20, 176], [45, 145], [25, 107], [38, 125], [49, 105]]

Sorted by first column, ascending order [[20, 176], [25, 107], [35, 182], [38, 125], [42, 103], [45, 145], [49, 105], [50, 141]]

Sorted by first column, descending order [[50, 141], [49, 105], [45, 145], [42, 103], [38, 125], [35, 182], [25, 107], [20, 176]]


Sorted by second column, ascending order [[42, 103], [49, 105], [25, 107], [38, 125], [50, 141], [45, 145], [20, 176], [35, 182]]

Sorted by second column, descending order [[35, 182], [20, 176], [45, 145], [50, 141], [38, 125], [25, 107], [49, 105], [42, 103]]

Original list unchanged [[35, 182], [50, 141], [42, 103], [20, 176], [45, 145], [25, 107], [38, 125], [49, 105]]