Cheshire is an SGML/Z39.50 engine being jointly developed by UC Berkeley and Liverpool University.
To simplify writing clients for it, Rob Sanderson <azaroth@liverpool.ac.uk> has built an implementation of ZOOM's Python binding based on Cheshire's low-level Z39.50 code.
It's (briefly) documented at gondolin.hist.liv.ac.uk/~cheshire/zoom/python.html and can be downloaded as a part of the full Cheshire package at ftp://cheshire.berkeley.edu/pub/cheshire/ (See the instructions on installing Cheshire.)
Cheshire also includes an implementation of the Tcl binding.