How to Create Nested Lists by Splitting Strings in Python

Published: 22 December 2025
on channel: vlogommentary

Learn how to split and group a sorted list of strings into nested sublists in Python, where each sublist starts and ends with single-element strings.
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This video is based on the question https://stackoverflow.com/q/79463169/ asked by the user 'matteo' ( https://stackoverflow.com/u/2509085/ ) and on the answer https://stackoverflow.com/a/79463251/ provided by the user 'SIGHUP' ( https://stackoverflow.com/u/17580381/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Problem Overview

Given a list of strings where each string is either a single element (like C1) or a pair separated by a comma (like C1,C2), the goal is to split this list into nested sublists. Each sublist should start and end with single elements, enclosing any pairs in between.

Example Input

[[See Video to Reveal this Text or Code Snippet]]

Desired Output

[[See Video to Reveal this Text or Code Snippet]]

Key Insight

Each nested list begins and ends with a single string (no commas).

The strings with commas belong to the sublist bounded by those single strings.

The input is sorted and guaranteed to start and end with a single string.

Concise Python Solution

[[See Video to Reveal this Text or Code Snippet]]

How It Works

Iterate through each string e in the list.

Append it to the current sublist.

When you hit a single string (',' not in e) and the current sublist has accumulated multiple elements, you close that sublist and start a new one.

In the end, remove any empty trailing sublist.

Output

[[See Video to Reveal this Text or Code Snippet]]

Summary

This method provides a clean and efficient way to generate nested sublists from a list of strings split by commas. It relies on the key property that single strings mark the boundaries of each group.

Use this approach when you have sorted strings representing segments or ranges defined by intermediary comma-separated pairs.

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