5 Best Ways to Convert Python Set to List

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💡 Problem Formulation: Python programmers often need to convert a set to a list for various reasons, such as to index elements, order them, or simply because a certain API expects a list argument rather than a set. For example, you may have a set {3, 1, 4} and want to turn it into a list like [3, 1, 4]. How can this be done efficiently and in a readable manner?

Method 1: Using the list() Constructor

The most straight-forward method to convert a set to a list is by using the list constructor list(), which takes an iterable as its argument and creates a new list containing the elements of that iterable.

Here’s an example:

my_set = {1, 2, 3}
my_list = list(my_set)

Output: [1, 2, 3]

This line of code takes the set my_set and passes it to the list() constructor, which then returns a new list containing the same elements. This method preserves the elements but might not preserve the order since sets are unordered collections.

Method 2: List Comprehension

List comprehension provides a concise way to create lists. A list comprehension consists of an expression followed by a for clause, then zero or more for or if clauses. In this case, it can be used to iterate over a set and create a list.

Here’s an example:

my_set = {3, 5, 9, 7}
my_list = [x for x in my_set]

Output: [9, 3, 5, 7]

The given code uses a list comprehension to iterate through each element x in my_set and places x into a new list. This method also does not guarantee the order of elements.

Method 3: Using the sorted() Function

If the order of elements is significant and you want to convert a set to a sorted list, you can use the sorted() built-in function, which returns a new sorted list from the items in the iterable.

Here’s an example:

my_set = {22, 55, 33, 44}
my_list = sorted(my_set)

Output: [22, 33, 44, 55]

This method not only converts the set to a list but also sorts the elements in ascending order. It’s a two-in-one approach when you need a sorted list.

Method 4: Add/Set Union Method

If you are dealing with a situation where your set is actually just a single element and you want to create a list out of it by combining it with another list, you might use the add or union set method combined with list concatenation.

Here’s an example:

my_element = 10
my_list = [1, 2, 3]
my_new_list = my_list + [my_element]

Output: [1, 2, 3, 10]

This method shows how to add a single element (not a set in this case) to an existing list. It presupposes that you want to maintain an existing list order and append a new value.

Bonus One-Liner Method 5: Using * Operator (Unpacking)

Python 3 allows argument unpacking using the * operator. This feature can be used to unpack a set directly into a list literal, which is particularly useful in function calls or combined data structures.

Here’s an example:

my_set = {'apple', 'banana', 'cherry'}
my_list = [*my_set]

Output: ['banana', 'apple', 'cherry']

This approach leverages the unpacking power of the * operator to turn a set directly into a list by “unpacking” its elements into a new list. As with other methods, it doesn’t maintain order.

Summary/Discussion

  • Method 1: Using the list() Constructor. It is quick, simple, and the most pythonic way. However, it does not preserve ordering.
  • Method 2: List Comprehension. Offers more control and can be modified to filter elements. Like method 1, the order is not preserved.
  • Method 3: Using the sorted() Function. Ideal when a sorted list is needed. More computationally expensive if sorting is not necessary.
  • Method 4: Add/Set Union Method. Useful for appending set elements to an existing list. Preserves the order of the original list.
  • Method 5: Using the * Operator. Versatile for function calls where arguments are needed in list form. Does not guarantee the order of the set.