To sort and return a Python list in a single line of code, use the
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.
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 list of 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.
a = [4, 2, 1, 3]
You want to sort this list and return the result in a single line. If you use the
list.sort() method, the return value is
print(a.sort()) # None
The return value of the
list.sort() method is
None, but many coders expect it to be the sorted list. So they’re surprised finding out that their variables contain the
None type rather than a sorted list.
None makes perfect sense for the
list.sort() method. Why? Because you call the method on a list object and 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:
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
sorted() function that takes an iterable and returns a new list with sorted elements.
a = [4, 2, 1, 3] # Method 1: sorted() print(sorted(a))
sorted() function generates a new sorted list that is put into the
print() function that prints the sorted list to the shell. The output is the sorted list:
[1, 2, 3, 4]
This method is the most Pythonic one. But are there alternatives?
Method 2: list.sort() + Ternary Operator
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
list.sort() method and the ternary operator!
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)
list.sort() and (2) the ternary operator:
list.sort()method sorts the list elements in place in an ascending manner. To customize the default sorting behavior, use the optional
keyargument by passing a function that returns a comparable value for each element in the list.
- The ternary operator
x if c else yconsists of three operands
y. It returns
xif the Boolean expression
True. Otherwise, if the expression
False, the ternary operator returns the alternative
The beautiful thing is that the one-liner
print(a if a.sort() else a) modifies the original list and returns it right away. How does it do this?
Explanation: First, the
a.sort() method is called to check which “branch” of the ternary operator should be visited. The return value of
a.sort() will always be
None value is automatically converted to the Boolean
False. Thus, the ternary operator always returns the list object referred to by variable
Note that the only purpose of the ternary operator is to make sure to call the
a.sort() method before returning the value
a—to make sure it is sorted!
If you print the original list to the shell, you’ll see that it is now sorted:
>>> 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
; to one-linerize a Python code snippet. This strategy works with flat Python programs without, possible nested, blocks:
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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While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.
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