How to Split Lists in Python?

Lists in Python are a powerful and versatile data structure. In many situations, we might need to split a list into smaller sublists for various operations such as processing data in chunks, grouping items or creating multiple lists from a single source. Let's explore different methods to split lists in Python.

Using List Slicing

The simplest way to split a list is by using slicing. This method allows you to divide a list into fixed-size chunks by specifying start and end indices. It is ideal when you know the desired size of each chunk.

# Splitting a list into two parts
li = [1, 2, 3, 4, 5, 6]

# Elements from index 0 to 2
a = li[:3]

# Elements from index 3 to end
b = li[3:]  

print(a, b)

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

Explanation: Slicing operator : is used to divide the list into two parts, with the first slice containing elements up to index 3 and the second slice starting from index 3.

Split list into chunks of equal size

For situations where you need to split a list into chunks of equal size, the list comprehension with slicing method can be used. This is particularly useful for large lists or when the exact size of chunks is known.

# Splitting a list into equal-sized chunks
li = [1, 2, 3, 4, 5, 6, 7, 8]

# Number of Chunks
n = 3

# Splitting Chunks and Storing them in List a
a = [li[i:i + n] for i in range(0, len(li), n)]

print(a)

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

Let's explore other methods of splitting lists in python:

Splitting Based on Conditions

For more control over splitting based on conditions, a loop can be used to separate elements based on custom logic.

# Splitting a list based on a condition
li = [1, 2, 3, 4, 5, 6, 7, 8]
a = []
b = []

for i in li:
  	
    # Keeping the even numbers in list a after splitting
    if i % 2 == 0:
        a.append(i)
        
    # Keeping the odd numbers in list b after splitting    
    else:
        b.append(i)

print(a, b)

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

Explanation: This approach separates the list into two sublists based on whether the elements are even or odd.

Using itertools

We can use itertools.islice for split the list. This is efficient and you don't need to create intermediate copies of list, unlike slicing and list comprehension.

from itertools import islice

a = [1, 2, 3, 4, 5, 6, 7, 8]
n = 3

# Create an iterator
it = iter(a)

# Use islice to generate chunks
chunks = []
for _ in range(0, len(a), n):
    chunks.append(list(islice(it, n)))

print(chunks)  # Output: [[1, 2, 3], [4, 5, 6], [7, 8]]

Using numpy (For Advanced Operations)

We can also use the numpy library for advanced operations. The array_split method in numpy allows splitting a list into a specified number of sublists, distributing elements as evenly as possible.

import numpy as np

# Splitting a list using numpy
li = [1, 2, 3, 4, 5, 6, 7]

# Number of Chunks
n = 3

# Keeping the chunks in list c after splitting
c = np.array_split(li, n)

print([list(c) for i in c])

[[array([1, 2, 3]), array([4, 5]), array([6, 7])], [array([1, 2, 3]), array([4, 5]), array([6, 7])], [array([1, 2, 3]), array([4, 5]), array([6, 7])]]

Explanation: The numpy.array_split function divides the list into sublists, ensuring all elements are distributed.