The PYMIDI Framework provides a set of classes and utility functions for communicating via MIDI, both with physical devices (MIDI interfaces, MIDI controllers, etc.) and with other MIDI software.
The framework is basically a wrapper around Apple's CoreMIDI framework, although you don't need to understand CoreMIDI to use the basics of PYMIDI. It provides a nicer Objective-C interface to basic CoreMIDI functionality and provides some additional functionality that is very useful for producing consistent and reliable handling of MIDI. It does not attempt to duplicate all of CoreMIDI's functionality in Objective-C; instead it allows advanced developers to access the underlying CoreMIDI data structures.
The framework sends and receives MIDI data, but it doesn't take care of parsing, processing or generating it.