Python - Ways to flatten a 2D list

Flattening a 2D list means converting a nested list structure into a single-dimensional list. Python provides several methods to accomplish this task, each with different performance characteristics and use cases.

Using itertools.chain.from_iterable()

The chain.from_iterable() function efficiently flattens a 2D list by chaining all sublists together ?

from itertools import chain

nested_list = [[1, 2, 3],
               [3, 6, 7],
               [7, 5, 4]]

print("Initial list:", nested_list)

# Converting 2D list into 1D using chain.from_iterable
flattened = list(chain.from_iterable(nested_list))
print("Flattened list:", flattened)

Initial list: [[1, 2, 3], [3, 6, 7], [7, 5, 4]]
Flattened list: [1, 2, 3, 3, 6, 7, 7, 5, 4]

Using List Comprehension

List comprehension provides a concise and readable way to flatten lists ?

nested_list = [[1, 2, 3],
               [3, 6, 7],
               [7, 5, 4]]

print("Initial list:", nested_list)

# Converting 2D list into 1D using list comprehension
flattened = [item for sublist in nested_list for item in sublist]
print("Flattened list:", flattened)

Initial list: [[1, 2, 3], [3, 6, 7], [7, 5, 4]]
Flattened list: [1, 2, 3, 3, 6, 7, 7, 5, 4]

Using functools.reduce()

The reduce() function applies a function cumulatively to combine all sublists ?

from functools import reduce

nested_list = [[1, 2, 3],
               [3, 6, 7],
               [7, 5, 4]]

print("Initial list:", nested_list)

# Converting 2D list into 1D using functools.reduce
flattened = reduce(lambda x, y: x + y, nested_list)
print("Flattened list:", flattened)

Initial list: [[1, 2, 3], [3, 6, 7], [7, 5, 4]]
Flattened list: [1, 2, 3, 3, 6, 7, 7, 5, 4]

Using sum() Function

The sum() function can concatenate lists when given an empty list as the start value ?

nested_list = [[1, 2, 3],
               [3, 6, 7],
               [7, 5, 4]]

print("Initial list:", nested_list)

# Converting 2D list into 1D using sum()
flattened = sum(nested_list, [])
print("Flattened list:", flattened)

Initial list: [[1, 2, 3], [3, 6, 7], [7, 5, 4]]
Flattened list: [1, 2, 3, 3, 6, 7, 7, 5, 4]

Comparison of Methods

Conclusion

For most cases, use list comprehension for its excellent readability. For performance-critical applications with large datasets, itertools.chain.from_iterable() is the fastest option. Avoid sum() for large lists due to performance overhead.