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

Flattening a list of lists into a single flat list can be achieved using various techniques in Python. Here are the most common methods:

1. Using List Comprehension

This is one of the simplest and most Pythonic methods to flatten a list of lists.

# List of lists
nested_list = [
    [1, 2, 3],
    [4, 5, 6],
    [7],
    [8, 9]
]

# Flatten the list using list comprehension
flat_list = [item for sublist in nested_list for item in sublist]

print(flat_list)

Output:

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

2. Using the `itertools.chain` Function

The itertools.chain function is a fast and efficient way to flatten a list of lists.

from itertools import chain

# List of lists
nested_list = [
    [1, 2, 3],
    [4, 5, 6],
    [7],
    [8, 9]
]

# Flatten the list using itertools.chain
flat_list = list(chain.from_iterable(nested_list))

print(flat_list)

Output:

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

3. Using `sum()` with an Empty List

The sum() function can concatenate lists when used with an empty list as the starting point.

# List of lists
nested_list = [
    [1, 2, 3],
    [4, 5, 6],
    [7],
    [8, 9]
]

# Flatten the list using sum()
flat_list = sum(nested_list, [])

print(flat_list)

Output:

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

Note: This method is less efficient for large lists as it creates intermediate lists during the summation.

4. Using a Nested Loop

This is a more verbose but straightforward approach.

# List of lists
nested_list = [
    [1, 2, 3],
    [4, 5, 6],
    [7],
    [8, 9]
]

# Flatten the list using a nested loop
flat_list = []
for sublist in nested_list:
    for item in sublist:
        flat_list.append(item)

print(flat_list)

Output:

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

5. Using `numpy` (if working with numerical data)

If you're working with numerical data, numpy provides an efficient way to flatten a list of lists.

import numpy as np

# List of lists
nested_list = [
    [1, 2, 3],
    [4, 5, 6],
    [7],
    [8, 9]
]

# Flatten the list using numpy
flat_list = np.concatenate(nested_list).tolist()

print(flat_list)

Output:

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

Which Method Should You Use?

Small/Medium Lists: Use list comprehension or itertools.chain for clarity and performance.
Large Lists: Use itertools.chain, as it is optimized for iterables.
Numerical Data: Use numpy if you already rely on numpy for other tasks.
Explicit Loops: Use a nested loop if readability is a priority and the list comprehension is too complex for others to understand.