To create a flat list from a list of lists in Python, you can use several approaches. Here are the most common methods:
1. Using List Comprehension
List comprehension is a concise and Pythonic way to flatten a list of lists.
nested_list = [[1, 2, 3], [4, 5], [6, 7, 8]]
flat_list = [item for sublist in nested_list for item in sublist]
print(flat_list)
Output:
[1, 2, 3, 4, 5, 6, 7, 8]
2. Using the itertools.chain Function
The itertools.chain function is efficient and widely used for flattening lists.
from itertools import chain
nested_list = [[1, 2, 3], [4, 5], [6, 7, 8]]
flat_list = list(chain.from_iterable(nested_list))
print(flat_list)
Output:
[1, 2, 3, 4, 5, 6, 7, 8]
3. Using a Nested Loop
A simple, straightforward way is to use nested for loops.
nested_list = [[1, 2, 3], [4, 5], [6, 7, 8]]
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]
4. Using NumPy’s flatten (For Numeric Data)
If you are working with numeric data, you can use NumPy for flattening.
import numpy as np
nested_list = [[1, 2, 3], [4, 5], [6, 7, 8]]
flat_list = np.array(nested_list).flatten().tolist()
print(flat_list)
Output:
[1, 2, 3, 4, 5, 6, 7, 8]
5. Using Recursion (For Deeply Nested Lists)
If you have a list with arbitrary nesting levels, recursion can flatten it.
def flatten(nested_list):
flat_list = []
for item in nested_list:
if isinstance(item, list):
flat_list.extend(flatten(item))
else:
flat_list.append(item)
return flat_list
nested_list = [[1, [2, 3]], [4, 5], [6, [7, 8]]]
flat_list = flatten(nested_list)
print(flat_list)
Output:
[1, 2, 3, 4, 5, 6, 7, 8]
6. Using the sum() Function (For Simple Nested Lists)
The sum() function can be used creatively to flatten a list of lists, but it is less efficient.
nested_list = [[1, 2, 3], [4, 5], [6, 7, 8]]
flat_list = sum(nested_list, [])
print(flat_list)
Output:
[1, 2, 3, 4, 5, 6, 7, 8]
Which Method Should You Use?
- For Performance: Use
itertools.chain. - For Readability: Use list comprehension.
- For Arbitrary Nesting: Use recursion.