How to Join a List of Lists? You Don’t. You Flatten It!

Short answer: To flatten your list of list lst, use the nested list comprehension [x for l in lst for x in l] to iterate over all nested lists (for l in lst) and over all their elements (for x in l). Each element x, you just place in the outer list, unchanged.

When browsing StackOverflow, I stumble upon this question from time to time: “How to Join a List of Lists?“. My first thought is that the asking person is referring to the join() function that converts an iterable (such as a list) to a string by concatenating its elements.

But nothing can be further from the truth! The question is usually about “flattening” the list: to transform a nested list of lists such as [[1, 2, 3], [4, 5, 6]] to a flat list [1, 2, 3, 4, 5, 6]. So, the real question is:

How to Flatten a List of Lists in Python?

Problem: Given a list of lists. How to flatten the list of lists by getting rid of the inner lists—and keeping their elements?

Example: You want to transform a given list into a flat list like here:

lst = [[2, 2], [4], [1, 2, 3, 4], [1, 2, 3]]

# ... Flatten the list here ...

print(lst)
# [2, 2, 4, 1, 2, 3, 4, 1, 2, 3]
Flatten a List of Lists with List Comprehension

Solution: Use a nested list comprehension statement [x for l in lst for x in l] to flatten the list.

lst = [[2, 2], [4], [1, 2, 3, 4], [1, 2, 3]]

# ... Flatten the list here ...
lst = [x for l in lst for x in l]

print(lst)
# [2, 2, 4, 1, 2, 3, 4, 1, 2, 3]

Explanation: In the nested list comprehension statement [x for l in lst for x in l], you first iterate over all lists in the list of lists (for l in lst). Then, you iterate over all elements in the current list (for x in l). This element, you just place in the outer list, unchanged, by using it in the “expression” part of the list comprehension statement [x for l in lst for x in l].

Try It Yourself: You can execute this code snippet yourself in our interactive Python shell. Just click “Run” and test the output of this code.

Exercise: How to flatten a three-dimensional list (= a list of lists of lists)? Try it in the shell!

Video: List Comprehension Python List of Lists

Watch the video to learn three ways how to apply list comprehension to a list of lists:

  • to flatten a list of lists,
  • to create a list of lists, and
  • to iterate over a list of lists.

Well, if you really want to learn how to join() a list in Python, visit this detailed tutorial on the Finxter blog.

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