pluggy is the crystallized core of plugin management and hook calling for
pytest. It enables 500+ plugins to extend and customize pytest's default
behaviour. Even pytest itself is composed as a set of pluggy plugins
which are invoked in sequence according to a well defined set of protocols.

It gives users the ability to extend or modify the behaviour of a host
program by installing a plugin for that program. The plugin code will run
as part of normal program execution, changing or enhancing certain aspects
of it.

In essence, pluggy enables function hooking so you can build "pluggable"
systems.