A validation error occurred when validating the instance document
The The usage of validation phases is a unique feature of ISO-Schematron and can be a very powerful tool e.g. for establishing validation stages or to provide different validators for different "validation audiences". Pass an Element Tree object to construct a XMLSchema validator: From version 2.3 on lxml features ISO-Schematron support built on the de-facto reference implementation of Schematron, the pure-XSLT-1.0 skeleton implementation.This is provided by the lxml.isoschematron package that implements the Schematron class, with an API compatible to the other validators'.If you ever need to pass an XPath as argument to the XSLT stylesheet you can pass in an etree.
A class which intends to use validated fields should sub-class itself from this class.Attributes are defined on the class which contains them using validators.Although validators can be defined by any client of this library, a number of standard validators are provided here.Apart from the built-in DTD support in parsers, lxml currently supports three schema languages: DTD, Relax NG and XML Schema.All three provide identical APIs in lxml, represented by validator classes with the obvious names.