Mercurial

Mercurial is a distributed open source source control (also known as "version control") system written in Python for tracking and handling file modifications. Mercurial can be used as the version control system for Python projects.

Mercurial is an implementation of the source control concept. Learn how these pieces fit together in the development environments chapter or view the table of contents for all topics.

Mercurial tutorials

The official Mercurial tutorial goes through the basics. It has great examples for syntax and expected output.
Getting started with Mercurial for version control seems mistitled because it is actually a short tutorial on how to build Mercurial extensions in Python. With a few lines of Python code it shows how to create your first extension and test it.
Mercurial: The Definitive Guide is a free online version of the O'Reilly book.
Monoroke is a Mercurial server written in Rust designed to be used for very large monorepos that have thousands of commits affecting millions of files per hour.