5 Best Approaches to Convert Set to List Python

5 Best Approaches to Convert Set to List Python

The set data type in Python is an unordered collection of unique, immutable objects. Sets can be beneficial to efficiently remove duplicates, test membership, and perform mathematical set operations. However, sometimes you need to convert those sets into lists for further processing or serialization.

In this post, we’ll clearly understand the differences between Python sets and lists, approaches to convert set to list Python, use cases where this conversion is necessary, best practices around these conversions, and related concepts new Python programmers should understand.

Python Lists vs Sets

First, let’s recap some key traits differentiating Python lists and sets built-in types:

  • Lists are ordered, indexed sequences allowing duplicate elements like [1, 1, 2, 3], while sets are unordered collections of distinct objects such as {3, 2, 1}.
  • Lists can hold arbitrary objects and data types together. Sets only hold a single data type – for example, a set of integers or a set of strings.
  • Sets efficiently test membership using hashes (O(1)) instead of linear scans (O(n)) in lists

This makes retrieving elements by index as well as maintaining insertion ordering possible with lists, unlike sets. On the flip side, sets provide faster membership tests and duplicate removal capabilities.

Read in more detail: Python List vs Sets

How to Convert Set to List Python?

Now let’s tackle techniques to convert set to list Python. Suppose we have a set of integers:

numbers_set = {1, 5, 2, 4, 5}

Approach 1: Python Set to List Using the list() function

We can directly use Python’s list() function to convert set to list python:


numbers_list = list(numbers_set)
print(numbers_list) # [1, 2, 4, 5]

Approach 2: By Iteration and Appending

Alternatively, we can iterate through our Python set and as we encounter each element we add that to our Python list, hence in the end we’ll have a list of all our elements that were earlier present in the set.


numbers_list = [] 
for num in numbers_set:
print(numbers_list) # [1, 2, 4, 5]

Approach 3: Using List Comprehensions

We can use list comprehension to create a new list from the elements in the set. This method is more concise and can be faster than manual iteration.


numbers_list = [num for num in numbers_set]
print(numbers_list) # [1,2,3,4]

Approach 4: Unpacking the Set inside parentheses

This method involves unpacking the set inside a list literal, which is created due to the presence of a single comma. This approach is faster but suffers from readability


numbers_list = [*numbers_set]
print(numbers_list) # [1,2,3,4]

Approach 5: Using the map function

We can use the map() function to convert set to list Python by passing the set as an argument to the map() function and returning a list of the elements.


numbers_list = list(map(lambda x: x, numbers_set))
print(numbers_list) # [1,2,3,4]

Use Cases Driving Set-List Conversions

Some common scenarios where converting a Python set into a list becomes necessary:

  • Serializing the collection to JSON or CSV which requires value ordering
  • Passing the data into a function expecting iterable arguments
  • Sorting or manually manipulating list elements (sets are unordered)
  • Maintaining insertion sequence along with membership testing abilities

For these use cases, sets provide efficient building blocks leveraging their uniqueness and membership testing. Yet interoperability with other functions needs ordered listings occasionally.

Best Practices for Smooth Conversions

Follow these best practices when converting back and forth between sets and lists in Python to avoid surprises:

  • Be aware ordering and duplicates get lost when casting lists to sets
  • Watch out for mixing data types if converting heterogeneous lists
  • Use list() or the list constructor instead of less readable alternatives
  • Note that nested objects in sets also require hashing all fields

And there you have it – a quick primer on effectively converting Python sets into lists using intuitive built-in functions as per application requirements!

While on the topic of Python sets, let’s also briefly mention related concepts that are handy to know:

1. Set Unions

Set unions create a new set with elements from two sets without overlap.


set_a = {1, 2, 3}
set_b = {3, 4, 5}

set_c = set_a | set_b # Set union => {1, 2, 3, 4, 5}

2. Set Intersections

Set intersections return common elements present in two sets:


set_a = {1, 2, 3}  
set_b = {2, 3, 4}

set_c = set_a & set_b # Set intersection => {2, 3}

3. Frozensets

Frozensets are immutable variants of standard sets in Python. This means elements can’t be added or removed once initialized like sets.


my_frozen_set = frozenset([1, 'Hi', True])


To sum up, the set and list data types have key differences in Python – sets are unordered collections of distinct elements supporting efficient membership testing, while lists maintain ordering and allow duplicates. Converting a set to list makes the elements sequence-ordered, serializable via JSON and sortable, while losing uniqueness.

Common Pythonic ways to convert a set to a list include direct type conversion using the list() method or list constructor call on the sets. Certain use cases like serializing data, passing as function arguments, sorting/processing require set ordering and duplication which lists enable.

It’s best practice in Python to type cast cleanly via list() and be aware ordering/duplication changes. Additionally, frozensets provide immutable variants of sets in Python.

Also Read:


  1. How do you convert a set to a list in Python?

    You can convert a set to a list in Python by using the list() method or list constructor.

  2. What is the difference between a set and a list in Python?

    Sets are unordered collections of unique elements while lists maintain ordering and can contain duplicate elements. Sets also support fast membership testing using hashes.

  3. Why would you need to convert a set to a list in Python?

    Common reasons to convert a set to a list include needing ordering to serialize data, pass into a function expecting a list, sort elements, manipulate items by index, or work with duplicate values.

  4. What happens to duplicates when converting a set to a list?

    Since Python sets only contain unique elements, any duplicates are collapsed when converting to a list. The result will be a list with just one instance of each distinct item from the original set.

  5. How can sets have unique elements while allowing different data types?

    Set elements have to be hashable datatypes. So you can have a set with an int and a string, but generally sets contain values all of the same basic immutable type like ints, strings, tuples etc.

  6. Does element ordering persist when converting a set into a list?

    No, since sets are inherently unordered by definition, converting a set to a list results in elements arranged in arbitrary order, not retaining any prior explicit or insertion ordering.

  7. What is a Python frozenset and how is it different from a set?

    A frozenset is an immutable, hashable ordered sequence of unique elements just like a set, but it cannot be changed after creation unlike sets.

  8. When should you use sets vs lists in Python?

    Use sets for faster membership tests, removing duplicates, and mathematical set operations. Use lists when you require ordering, indexes, varied data types within one structure.

  9. What are common set operations supported in Python?

    Python supports set unions, intersections, differences, and symmetric differences through operators like |, &, – and ^ respectively.

  10. How do you check if an item exists in a set vs a list?

    Use the in operator for readability, but it scans the entire list linearly while sets use highly optimized hash lookups to check membership.

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