How to Convert a List of Lists to a Set in Python? : Chris

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July 10, 2022
How to Convert a List of Lists to a Set in Python?
by: Chris
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Problem Formulation

💬 Question: Given a list of lists (=nested list) in Python. How to convert this list of lists to a set in Python?

This is not a trivial problem because Python lists are mutable and, therefore, not hashable. And because of that, they cannot be converted to a set using the direct set() approach like so:

my_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
my_set = set(my_list)

This raises the TypeError: unhashable type: 'list' as can be seen in the output when executing the previous code snippet:

Traceback (most recent call last):
  File "...\code.py", line 2, in <module>
    my_set = set(my_list)
TypeError: unhashable type: 'list'

Let’s have a look at different ways to resolve this problem and convert a nested list to a set in Python.

Method 1: Set Comprehension + tuple()

To convert a list of lists my_list to a set in Python, you can use the expression {tuple(x) for x in my_list} that goes over all inner lists in a set comprehension and converts each of them to a tuple. Tuples are immutable, hashable, and therefore can be used in sets, unlike lists.

Here’s the code example:

my_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
my_set = {tuple(x) for x in my_list}
print(my_set)
# {(7, 8, 9), (1, 2, 3), (4, 5, 6)}

Notice how the order of the elements is lost after converting the list of lists to a set!

Feel free to watch this video to learn more about set comprehension:

🌎 Learn More: You can learn more about set comprehension in this Finxter blog tutorial.

Method 2: Generator Expression + set() + tuple()

A variant of the previous approach is the following:

To convert a nested my_list to a set in Python, use the expression set(tuple(x) for x in my_list) that iterates over all inner lists in a generator expression passed into the set() function and convert each of them to a tuple using tuple() built-in function.

Again, tuples are immutable, hashable, and therefore can be used in sets, unlike lists.

my_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
my_set = set(tuple(x) for x in my_list)
print(my_set)
# {(7, 8, 9), (1, 2, 3), (4, 5, 6)}

Super concise one-liner, isn’t it?

You can learn more about generators in this video tutorial — feel free to watch to build your background knowledge in Python!

🌎 Learn More: You can learn more about generator expressions in this Finxter blog tutorial.

Method 3: Loop + Convert + Add Tuples

A simple and straightforward approach to convert a Python list of lists to a set is in four steps:

  1. Initialize an empty set using set(),
  2. Iterate over the inner list in a simple for loop,
  3. Convert the inner list to an immutable tuple using tuple(), and
  4. Add the newly-created tuple to the set using set.add().

Here’s the code example:

my_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

my_set = set()
for x in my_list:
    my_set.add(tuple(x))

print(my_set)
# {(7, 8, 9), (1, 2, 3), (4, 5, 6)}

🌎 Learn More: You can learn more about the set.add() method in this Finxter blog tutorial.

Method 4: Flatten List of Lists + Set Comprehension

If you need to flatten the nested list before converting it to a flat set, you can use the set comprehension {i for lst in my_list for i in lst} with nested one-line for loop expression to iterate over all inner elements of the list of lists.

Here’s the code:

my_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

my_set = {i for lst in my_list for i in lst}

print(my_set)
# {1, 2, 3, 4, 5, 6, 7, 8, 9}

Here’s a background video on flattening a list of lists:

🌎 Learn More: You can learn more about how to flatten a list of lists in this Finxter blog tutorial.

Method 5: Convert Inner Lists to Strings

An alternative approach to convert a list of lists to a set is to convert the inner lists to strings because strings are immutable and hashable and can therefore be used as set elements:

{str(lst) for lst in my_list}

We use the str() function in a set comprehension statement to convert the list of lists to a set of strings.

Here’s the code example:

my_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

my_set = {str(lst) for lst in my_list}

print(my_set)
# {'[7, 8, 9]', '[1, 2, 3]', '[4, 5, 6]'}

🌎 Learn More: If you want to convert the string representation of the list back to a list, you can find out more in this Finxter tutorial.

Summary

The five most Pythonic ways to convert a list of lists to a set in Python are:

Feel free to check out more free tutorials and cheat sheets for learning and improving your Python skills in our free email academy:

Nerd Humor

Oh yeah, I didn’t even know they renamed it the Willis Tower in 2009, because I know a normal amount about skyscrapers. — xkcd (source)

July 10, 2022 at 01:58PM
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