I've noticed a difference in behaviour between versions 3.4.5 and 4.0.0, that I couldn't read myself to understanding. Either I stumbled upon an unintentional bug, or this becomes a request to clarify the difference between major version 3 and 4 of this library.

I'm quite the amateur when it comes to XML Schema details. My objective is to produce XML that conforms to a schema specified by a service provider. Since version 4.0.0, the schema I use to validate against is no longer valid - it can't be parsed by this library anymore.

I have two workarounds:

• pin the version of xmlschema to 3.4.5
• use version 4.0.0 but add validation='lax' when loading the schema(s)

The error I get is this:

>           raise error
E           xmlschema.validators.exceptions.XMLSchemaParseError: selector xpath expression can only select elements:
E           
E           Schema component:
E           
E             <xs:keyref xmlns:xs="http://www.w3.org/2001/XMLSchema" name="fk-mainLocation" refer="key-location">
E               <xs:selector xpath="." />
E               <xs:field xpath="@mainLocation" />
E             </xs:keyref>
E           
E           Path: /xs:schema/xs:element/xs:complexType/xs:sequence/xs:element/xs:keyref
E           
E           Schema URL: file:///Users/....../edumedia/schema-files/xml-import.xsd

The Schema URL file is a local copy of this: https://github.com/keghub/xml-import/blob/master/schemas/3.0/xml-import.xsd

Here's a repo to quickly reproduce this difference between 3.4.5 and 4.0.0: https://github.com/FredrikWendt/xmlschema-issue

Is this change in behaviour by design? I looked through the CHANGELOG to look for changes, but I don't see it. While I'm fully aware moving to 4.x is a major release, I would love/appreciate some notes describing changes from 3 to 4.