Scimitar is an implementation of ISO Schematron that compiles a Schematron schema into a Python validator script, making it a faster and somewhat more flexible approach than the usual XSLT implementations.
Schematron is an XML schema language in which you express a set of rules that the document must meet, rather than expressing a full grammar for the XML vocabulary (which is the more common approach to XML schemata). It is by far the most flexible XML schema language available.
Scimitar support all of the Schematron 1.5 subset except for keys. See the TODO file for gaps in Scimitar functionality and convenience, which are being worked on.
Scimitar is open source, provided under the 4Suite variant of the Apache license.
The compiler program runs standalone on Python 2.2 or more recent, although if you are using an earlier version than 2,3, you must also install Optik 1.4.1 or more recent. In addition to the above requirements the generated validators require 4Suite 1.0a3 or more recent (really only tested with latest 4Suite CVS).