How do I make a flat list out of a list of lists?

list

python

How do I make a flat list out of a list of lists?

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In Python, you can flatten a list of lists into a single flat list using various methods. Here are some of the most common approaches:

1. Using a List Comprehension

List comprehensions provide a concise way to flatten a list of lists:

python

1list_of_lists = [[1, 2, 3], [4, 5], [6, 7, 8]]
2flat_list = [item for sublist in list_of_lists for item in sublist]
3
4print(flat_list)

Output:

python

[1, 2, 3, 4, 5, 6, 7, 8]

2. Using `itertools.chain`

The itertools.chain function is another efficient way to flatten a list of lists:

python

1import itertools
2
3list_of_lists = [[1, 2, 3], [4, 5], [6, 7, 8]]
4flat_list = list(itertools.chain(*list_of_lists))
5
6print(flat_list)

Output:

python

[1, 2, 3, 4, 5, 6, 7, 8]

Alternatively, you can also use itertools.chain.from_iterable which avoids the unpacking of the list:

python

flat_list = list(itertools.chain.from_iterable(list_of_lists))

3. Using `sum()`

Although less common, you can use sum() with an empty list as the start value:

python

1list_of_lists = [[1, 2, 3], [4, 5], [6, 7, 8]]
2flat_list = sum(list_of_lists, [])
3
4print(flat_list)

Output:

python

[1, 2, 3, 4, 5, 6, 7, 8]

4. Using `functools.reduce`

You can also use functools.reduce to flatten a list of lists:

python

1from functools import reduce
2
3list_of_lists = [[1, 2, 3], [4, 5], [6, 7, 8]]
4flat_list = reduce(lambda x, y: x + y, list_of_lists)
5
6print(flat_list)

Output:

python

[1, 2, 3, 4, 5, 6, 7, 8]

5. Using `numpy` (if your lists are numerical and you have NumPy installed)

If you're dealing with numerical data, you can use numpy's flatten method:

python

1import numpy as np
2
3list_of_lists = [[1, 2, 3], [4, 5], [6, 7, 8]]
4flat_list = np.array(list_of_lists).flatten().tolist()
5
6print(flat_list)

Output:

python

[1, 2, 3, 4, 5, 6, 7, 8]

Summary

List Comprehension: Simple and Pythonic.
itertools.chain: Efficient for large lists.
sum(): Simple but less efficient for large lists.
functools.reduce: Less common, but a functional approach.
numpy.flatten: Great for numerical data, but requires NumPy.

The best method depends on your specific use case, but list comprehension and itertools.chain are generally the most common and efficient ways to flatten a list of lists in Python.