How to partition a list into n nearly equal parts in Python

Divide a list into n contiguous chunks of nearly equal size using an average-length calculation that distributes the remainder evenly.

Easy Python 3.9+ Aug 9, 2026 Algorithms & data structures 14 views 0 copies

partitioning chunks slicing algorithms

Python code

28 lines

Python 3.9+

def partition(lst, n):
    """Partition a list into n nearly equal contiguous parts."""
    if n <= 0:
        raise ValueError("n must be positive")
    if not lst:
        return [[] for _ in range(n)]
    
    parts = []
    avg = len(lst) / n
    last_idx = 0.0
    
    while last_idx < len(lst):
        end_idx = int(round(last_idx + avg))
        parts.append(lst[int(last_idx):end_idx])
        last_idx = end_idx
    
    # Ensure we return exactly n parts
    while len(parts) < n:
        parts.append([])
    
    return parts


if __name__ == "__main__":
    data = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
    for n in range(1, 6):
        result = partition(data, n)
        print(f"n={n}: {result}")

Output

stdout

n=1: [[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]]
n=2: [[1, 2, 3, 4, 5], [6, 7, 8, 9, 10]]
n=3: [[1, 2, 3, 4], [5, 6, 7], [8, 9, 10]]
n=4: [[1, 2, 3], [4, 5, 6], [7, 8], [9, 10]]
n=5: [[1, 2], [3, 4], [5, 6], [7, 8], [9, 10]]

How it works

The function computes the average chunk size as len(lst) / n and uses a floating-point boundary last_idx to decide where each chunk ends. By rounding the cumulative average, longer parts appear early in the list and shorter parts later, keeping sizes within one element of each other. The final while loop guarantees the result has exactly n parts, even when the list length is smaller than n or zero. This approach is efficient—O(n) time—and avoids sorting or complex index math.

Common mistakes

Using integer division (//) for avg, which truncates and loses the remainder distribution
Forgetting to check for n <= 0, causing division by zero or infinite loops
Not padding with empty lists when n exceeds the list length
Assuming parts must be equal size when the list length is not divisible by n

Variations

Use a simple slice-based approach: `[lst[i::n] for i in range(n)]` for non-contiguous parts
For equal-size partitions with remainder handled differently, use `[lst[i::n] for i in range(n)]` but note it groups by element index not contiguous blocks

Real-world use cases

Splitting a dataset into train/validation/test folds for cross-validation with near-balanced classes.
Distributing a list of tasks across multiple worker processes or threads for parallel execution.
Paginating long result lists into sequential pages of approximate equal size for API responses.

How to partition a list into n nearly equal parts in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

More from Algorithms & data structures

Tutorials

Quizzes

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

More from Algorithms & data structures

Keep learning

Tutorials

Quizzes