[Tut] How to Sort and Return a Python List in One Line?

[Tut] How to Sort and Return a Python List in One Line? - Printable Version

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[Tut] How to Sort and Return a Python List in One Line? - xSicKxBot - 09-09-2020

How to Sort and Return a Python List in One Line?

To sort and return a Python list in a single line of code, use the

Code:
sorted(list)

method that returns a new list of sorted elements. It copies only the references to the original elements so the returned list is not a deep but a shallow copy.

[Image: sortandreturn-1024x576.jpg]

Let’s dive into the challenge to learn about more details and alternatives. Everything is not always simple. By studying the different methods that solves this, you’ll become a better coder!

Problem: Given a listof comparable objects such as integers or floats. Is there a way to sort the list and return the sorted list in a single line of Python code?

Example: Say, you’ve got the following list.

Code:
a = [4, 2, 1, 3]

You want to sort this list and return the result in a single line. If you use the

Code:
list.sort()

method, the return value is

Code:
None

Code:
print(a.sort())
# None

The return value of the

Code:
list.sort()

method is

Code:
None

, but many coders expect it to be the sorted list. So they’re surprised finding out that their variables contain the

Code:
None

type rather than a sorted list.

However, returning

Code:
None

makes perfect sense for the

Code:
list.sort()

method. Why? Because you call the method on a list objectand it modifies this exact list object. It doesn’t create a new list—there won’t be a new list object in memory.

So, how to sort and return a list in a single line of Python code? As a rule of thumb, there are always multiple ways to accomplish the same thing in Python. Let’s dive into the different ways to accomplish this!

Here’s a quick overview of the methods addressed in this article:

https://repl.it/@finxter/ModestAllMp3?lite=true

Exercise: Change the list to be sorted by adding negative floats. Does it still work?

You’ll now learn more about each of the methods.

Method 1: sorted()

The easiest way to accomplish this task is to call Python’s built-in

Code:
sorted()

function that takes an iterable and returns a new list with sorted elements.

Code:
a = [4, 2, 1, 3] # Method 1: sorted()
print(sorted(a))

The

Code:
sorted()

function generates a new sorted list that is put into the

Code:
print()

function that prints the sorted list to the shell. The output is the sorted list:

Code:
[1, 2, 3, 4]

This method is the most Pythonicone. But are there alternatives?

Method 2: list.sort() + Ternary Operator

The

Code:
sorted()

method leaves the original list unchanged. But what if you want to sort the original list and get this original list as an output that you can assign to a variable?

The answer is simple: use a combination of the

Code:
list.sort()

method and the ternary operator!

Code:
a = [4, 2, 1, 3] # Method 2: list.sort() + ternary
print(a if a.sort() else a)
# [1, 2, 3, 4]

You need to understand two concepts: (1)

Code:
list.sort()

and (2) the ternary operator:

The
Code:
list.sort()
method sorts the list elements in place in an ascending manner. To customize the default sorting behavior, use the optional
Code:
key
argument by passing a functionthat returns a comparable value for each element in the list.
The ternary operator
Code:
x if c else y
consists of three operands
Code:
x
,
Code:
c
, and
Code:
y
. It returns
Code:
x
if the Boolean expression
Code:
c
evaluates to
Code:
True
. Otherwise, if the expression
Code:
c
evaluates to
Code:
False
, the ternary operator returns the alternative
Code:
y
.

The beautiful thing is that the one-liner

Code:
print(a if a.sort() else a)

modifies the original list and returns it right away. How does it do this?

Explanation: First, the

Code:
a.sort()

method is called to check which “branch” of the ternary operator should be visited. The return value of

Code:
a.sort()

will always be

Code:
None

. The

Code:
None

value is automatically converted to the Boolean

Code:
False

. Thus, the ternary operator always returns the list object referred to by variable

Code:
a

.

Note that the only purpose of the ternary operator is to make sure to call the

Code:
a.sort()

method before returning the value

Code:
a

—to make sure it is sorted!

If you print the original list to the shell, you’ll see that it is now sorted:

Code:
>>> print(a)
[1, 2, 3, 4]

Method 3: Combining Multiple Statements in a Single Line with Semicolon

An alternative is chaining the statements with a semicolon

Code:
;

to one-linerize a Python code snippet. This strategy works with flat Python programs without, possible nested, blocks:

Code:
a = [4, 2, 1, 3] # Method 3: semicolon
a.sort(); print(a)
# [1, 2, 3, 4]

If you need to sort a Python list and print its return value to the shell—for example because you run this command from the command line or terminal—you can use this excellent strategy.

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