Skip to main content

Sorting

Sort items in a list in alphabetical order, or reverse alphabetical order.

lowercase and uppercase letters may affect the sort order.

sort() method​

Changes the order of a list permanently

lst.sort()
lst.sort(reverse=True)

sorted() function​

Returns a copy of the list, leaving the original list unchanged.

print(sorted(lst))
print(sorted(lst, reverse=True))
# Guaranteed to be stable

Reversing the order of a list​

lst.reverse()

2-D list sort with given column​

students.sort(key=lambda x:x[1])

Sorting a dictionary with value in descending order and if equal sort by key in ascending order​

dic = {'z': 5, 'b': 2, 'a': 2}
{k:v for k, v in sorted(dic.items(), key=lambda item: (-item[1], item[0]))}
{'z': 5, 'a': 2, 'b': 2}

Custom sorting with key=​

For more complex custom sorting, sorted() takes an optional "key=" specifying a "key" function that transforms each element before comparison. The key function takes in 1 value and returns 1 value, and the returned "proxy" value is used for the comparisons within the sort.

For example with a list of strings, specifying key=len (the built in len() function) sorts the strings by length, from shortest to longest. The sort calls len() for each string to get the list of proxy length values, and then sorts with those proxy values.

strs = ['ccc', 'aaaa', 'd', 'bb']
print sorted(strs, key=len) ## ['d', 'bb', 'ccc', 'aaaa']

image

As another example, specifying "str.lower" as the key function is a way to force the sorting to treat uppercase and lowercase the same:

## "key" argument specifying str.lower function to use for sorting
print sorted(strs, key=str.lower) ## ['aa', 'BB', 'CC', 'zz']

You can also pass in your own MyFn as the key function, like this

## Say we have a list of strings we want to sort by the last letter of the string
strs = ['xc', 'zb', 'yd' ,'wa']
## Write a little function that takes a string, and returns its last letter.
## This will be the key function (takes in 1 value, returns 1 value).
def MyFn(s):
    return s[-1]

## Now pass key=MyFn to sorted() to sort by the last letter:
print sorted(strs, key=MyFn)  ## ['wa', 'zb', 'xc', 'yd']