How to Flatten a List of Lists in Python with One Line

[TIPS] Python: Flatten a List of Lists in One Line

When working with Python, you’ll often encounter this common situation:

You have a list of lists, and you want to flatten it into a single list.

The Problem

Here’s an example:

data = [[1, 2], [3, 4], [5, 6]]

Desired output:

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

The Simple Solution: List Comprehension

The most Pythonic way to flatten a list of lists is:

flattened = [item for sublist in data for item in sublist]

Explanation

This is a nested list comprehension, which is functionally equivalent to:

flattened = []

for sublist in data:

    for item in sublist:

        flattened.append(item)

But in a much more concise form.

Alternative: Using itertools.chain

For larger or memory-sensitive datasets, you can use Python’s built-in itertools module:

import itertools

flattened = list(itertools.chain.from_iterable(data))

This approach creates an iterator, which can be more efficient when working with large amounts of data.

Example

data = [['a', 'b'], ['c', 'd'], ['e']]

flattened = [item for sublist in data for item in sublist]

print(flattened)

# Output: ['a', 'b', 'c', 'd', 'e']

Best Practice

Use list comprehension for clear and concise code when working with small to medium datasets.
Use itertools.chain when working with large data or when you want to process items lazily.

This technique is a staple in clean Python development and is widely used in data manipulation, web scraping, and data pipelines.

Reference: StackOverflow’s top answer