How to Flatten a List of Lists in Python?

Transforming a List of Lists into a Flat List

In some situations, dealing with lists can be rather complicated if they entail nested structures, such as a "list of lists." To address this issue, the task at hand involves transforming a list of lists into a simplified, flat list encompassing the elements from all the nested lists.

Consider the example of a list of lists:

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

The goal is to yield a flat list containing all the individual elements:

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

There are two primary approaches to achieving this transformation:

Method 1: Nested List Comprehension

A comprehensive list comprehension effectively flattens the list structure by iterating through the list of lists (xss) and individual lists (xs) to append elements to the flat list:

flat_list = [x for xs in xss for x in xs]

Method 2: Using the Flatten Function

Another efficient technique is to define a flatten function utilizing the same iterators:

def flatten(xss):
    return [x for xs in xss for x in xs]

Both methods yield the desired flat list from the original list of lists.

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