Python List Sort Method

title: List Sort Method

List Sort Method

Python lists have a built-in sort() method that modifies the list in-place and a sorted() built-in function that builds a new sorted list from an iterable.

list.sort(key=…, reverse=[True/False])


There are two optional parameters to this method:

key – The input value for the key parameter should be a function that takes a single argument and returns a value used for comparisons to sort the items in the list.


value=True : Sorts the items in the list in descending order.

value=False : Sorts the items in the list in ascending order. This is considered the default value.

Please note that the sort() method does not return any value. It modifies the original list.

Example Usage

a = [4, 2, 5, 3, 1] a.sort() print a # prints [1, 2, 3, 4, 5] b = ['free', 'code', 'camp'] b.sort() print b # prints ['camp', 'code', 'free']

Consider an example with the reverse parameter:

a = [4, 2, 5, 3, 1] #Sorts the list in descending order a.sort(reverse=True) print a # prints [5, 4, 3, 2, 1]

If you want to sort the list based on your own function, then use the key parameter.

Here is an example to sort the strings in the list by length, in ascending order:

a = ["hello", "hi", "hey"] #The built-in len() function is given as an input to key parameter to sort the strings by length a.sort(key = len) print a # prints ['hi', 'hey', 'hello']

Here is another example, where the list contains tuples(name, age).
The usage below shows how to sort the list by age, in ascending order:

#Consider the second element in the tuple for sorting >>> def compareByAge(element): ... return element[1] b = [('Adam', 20), ('Rahman', 30), ('Rahul', 25)] #Sort the list by age b.sort(key = compareByAge) #Output print b # prints [('Adam', 20), ('Rahul', 25), ('Rahman', 30)]

You can also use Lambda expression instead of full function to define key.
Here is example of sorting strings based on last two characters:

# Our strings can contain any characters and we want to sort them based on last two of them strings = ["apple_05", "orange_01", "strawberry_03", "pear_04", "banana_02"] # Take just last two characters as key value strings.sort(key = lambda x: x[-2:]) # Output print strings # Prints ['orange_01', 'banana_02', 'strawberry_03', 'pear_04', 'apple_05']

Sorting Basics

A simple ascending sort is very easy — just call the sorted() function. It returns a new sorted list:

>>> sorted([5, 2, 3, 1, 4]) [1, 2, 3, 4, 5]

You can also use the list.sort() method of a list. It modifies the list in-place (and returns None to avoid confusion). Usually it’s less convenient than sorted() – but if you don’t need the original list, it’s slightly more efficient:

>>> a = [5, 2, 3, 1, 4] >>> a.sort() >>> a [1, 2, 3, 4, 5]

Another difference is that the list.sort() method is only defined for lists. In contrast, the sorted() function accepts any iterable:

>>> sorted({1: 'D', 2: 'B', 3: 'B', 4: 'E', 5: 'A'}) [1, 2, 3, 4, 5]

Implementation Details

If one wants to know details about the implementation of the sort function, the algorithm, and the time complexity, etc, refer here. In brief, sort function uses TimSort algorithm, which according to Python Developers, is :-

an adaptive, stable, natural mergesort, modestly called
timsort (hey, I earned it ). It has supernatural performance on many
kinds of partially ordered arrays (less than lg(N!) comparisons needed, and
as few as N-1), yet as fast as Python’s previous highly tuned samplesort
hybrid on random arrays.

sort() Parameters

By default, sort() doesn’t require any extra parameters. However, it has two optional parameters:

  • reverse – If true, the sorted list is reversed (or sorted in Descending order).
  • key – A function that serves as a key for the sort comparison.

More Information:

More information about sort() can be found here.

More information about sort() and sorted() can be found here.

More information about sort() and sorted() can be found here.

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