Tyrannioware is Aaron Lav's bibliographic cataloguing suite, written in Python. One of things it does is fetch MARC records from the Library of Congress using Z39.50, so Aaron separated out the Z39.50 component of his software behind a ZOOM API, and has made it generally available.
It is a pure Python project: the ASN.1 compiler, the BER encoder and decoder and everything else is written from the ground up in Python: the low-level libraries are also available. As if that weren't enough, versions from 0.9 onwards also contain a graphical Zthes browser, which was written to test the python GRS-1 implementation.
The Tyrannioware ZOOM implementation is available at http://www.pobox.com/~asl2/software/PyZ3950/zoom.html