Viper is a SourceMod extension that embeds the CPython interpreter and loads Python plug-ins. It has a standard library written completely in C++ for excellent performance.
Learn the basics of Viper plug-in writing with My First Viper Plug-in. Learn how to access the Viper menu from the console in The Viper Menu. See all the global forwards (such as client_connect or OnClientConnect in SourceMod) provided by Viper in Global Forwards.