**π‘ Problem Formulation:** Python developers often encounter situations where they have a list of lists, and they need to flatten it into a single list. For example, if the input is `[[1, 2], [3, 4], [5, 6]]`

, the desired output is a single list: `[1, 2, 3, 4, 5, 6]`

. Appending lists of lists into one list is a common operation that can be accomplished in multiple ways in Python.

## Method 1: Using a For Loop

This method involves iterating over each sublist within the list of lists and appending the elements of each sublist to a new list. It’s simple, easy to understand, and does not require any additional functions or methods from Python’s standard library.

Here’s an example:

result = [] list_of_lists = [[1, 2], [3, 4], [5, 6]] for sublist in list_of_lists: result.extend(sublist)

Output: `[1, 2, 3, 4, 5, 6]`

This code snippet declares an empty list named `result`

and iterates over each sublist in the `list_of_lists`

. The `extend()`

method then appends each element of the sublist to the `result`

, producing a flattened list.

## Method 2: List Comprehension

List comprehension offers a more concise and Pythonic way to flatten a list of lists. It’s often considered more readable for those who are familiar with Python’s syntactic sugar.

Here’s an example:

list_of_lists = [[1, 2], [3, 4], [5, 6]] flattened_list = [element for sublist in list_of_lists for element in sublist]

Output: `[1, 2, 3, 4, 5, 6]`

In the example above, we create `flattened_list`

by looping through each element of each sublist in a single line of code. The list comprehension iterates through each sublist, and then each element in that sublist, adding the element to the new list.

## Method 3: Using itertools.chain

The `itertools.chain`

function is part of Python’s standard library, which is used for treating consecutive sequences as a single sequence. It’s useful for efficient and fast iteration over the elements.

Here’s an example:

from itertools import chain list_of_lists = [[1, 2], [3, 4], [5, 6]] flattened_list = list(chain.from_iterable(list_of_lists))

Output: `[1, 2, 3, 4, 5, 6]`

The given code snippet imports `chain`

from the `itertools`

module and then uses its `from_iterable`

method to iterate over each element of the provided list of lists. This is converted to a single list with the `list()`

function.

## Method 4: Using NumPy

For larger datasets or for applications involving numerical computing, the NumPy library provides efficient array operations. The `numpy.concatenate()`

method can be used to flatten a list of lists.

Here’s an example:

import numpy as np list_of_lists = [[1, 2], [3, 4], [5, 6]] flattened_list = list(np.concatenate(list_of_lists))

Output: `[1, 2, 3, 4, 5, 6]`

After importing the NumPy package, the example utilizes `np.concatenate()`

to merge the sublists into one array and then converts the resultant array back into a list with the `list()`

function.

## Bonus One-Liner Method 5: Using sum

The built-in `sum()`

function can be used in a creative way to flatten a list of lists. This method uses `sum()`

to concatenate lists, with an initial empty list as the starting value.

Here’s an example:

list_of_lists = [[1, 2], [3, 4], [5, 6]] flattened_list = sum(list_of_lists, [])

Output: `[1, 2, 3, 4, 5, 6]`

This code snippet passes two arguments to the `sum()`

function: our list of lists and an empty list. The second argument acts as the initializer, and `sum()`

effectively appends each sublist to this initializer to produce a single flattened list.

## Summary/Discussion

**Method 1: Using a For Loop.**Straightforward. Ideal for beginners. Supports complex nesting. May be slower for large datasets.**Method 2: List Comprehension.**Pythonic and concise. Readable for those familiar with the construct. Potentially less efficient for very large lists.**Method 3: Using itertools.chain.**Part of the standard library. Efficient and suitable for large or complex iterables. Requires importing a module.**Method 4: Using NumPy.**Fast performance for large numeric datasets. Requires NumPy installation. May not be necessary for simple/small lists.**Method 5: Using sum.**One-liner. Straightforward, but may be less intuitive that it can be used for lists. Not recommended for very large lists as it can be less efficient.