How Do I Flatten a Nested List in Python?

Flattening Lists: A Step-by-Step Guide

Nested lists can arise when dealing with complex data structures or working with data that has been grouped in various ways. While these lists can be useful for organization, there are times when we need to flatten them into a single, one-dimensional list for further processing or analysis.

The solution to flattening a list of lists lies in the concept of unpacking the nested structures. One effective method involves using a nested list comprehension:

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

In this approach, we iterate through each sublist in xss and then iterate through each element in the sublist, appending each element to the new, flattened list. This technique provides a straightforward way to obtain the desired result.

Alternatively, we can implement the same logic using a more imperative approach:

def flatten(xss):
    flat_list = []
    for xs in xss:
        for x in xs:
            flat_list.append(x)
    return flat_list

This function, while more verbose, offers greater flexibility in handling potential variations in the nested list structure.

It's worth noting that other flattening techniques exist, each with its own advantages and drawbacks. However, the nested list comprehension approach and the flatten function outlined above are generally considered to be efficient and simple to implement.

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