XPATH assertion in xsd to check value of an attribute of first and last element of a complex type - xsd-validation

Question

I have the following xsd model and I need to have assertions to check on the type of first and last element of kPartsList : last part must be castOffPartPoint and first part must be castOnPartSeg;
I am using Python Package xmlSchema to validate the xml document
<?xml version="1.1" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
targetNamespace = "http://www.ludd21.com/kPartModel"
xmlns = "http://www.ludd21.com/kPartModel"
elementFormDefault="qualified"
vc:minVersion = "1.1"
xpathDefaultNamespace="##targetNamespace"
xmlns:vc="http://www.w3.org/2007/XMLSchema-versioning"
>
<xs:element name="kPartModel">
<xs:complexType>
<xs:sequence>
<xs:element ref="kPartsPiece" minOccurs="1" maxOccurs = "unbounded"/>
</xs:sequence>
<xs:attribute name="modelName" type="xs:NCName" use = "required"/>
<xs:attribute name= "otherattribute" type="xs:string" default = ""/>
</xs:complexType>
<!--piecename must be unique within kpModel-->
<xs:unique name= "kPartModel">
<xs:selector xpath="*"/>
<xs:field xpath= "#pieceName"/>
</xs:unique>
</xs:element>
<xs:element name="kPartsPiece">
<xs:complexType>
<xs:sequence>
<xs:element ref= "kPartsList"/>
</xs:sequence>
<xs:attribute name="pieceName" type="xs:NCName"/>
<!-- #partNumber is unique across kPartsList -->
<xs:unique id = "unique-partNumber" name= "unique-partNumber">
<xs:selector xpath="*/*"/>
<xs:field xpath= "#partNumber"/>
</xs:unique>
</xs:element>
</xs:complexType>
<xs:element name = "kPartsList" >
<xs:complexType>
<xs:sequence>
<xs:choice minOccurs= "0" maxOccurs = "unbounded">
<xs:element ref = "castOnPartSeg" />
<xs:element ref = "joinPart"/>
<xs:element ref = "castOffPartPoint"/>
</xs:choice>
</xs:sequence>
<!-- last part must be castOff -->
<xs:assert id = "test-end-castOff" test = "(kPartsList[last()]/#type = 'castOffPartPoint') or (kPartsList[last()]/#type = 'castOffPartSeg')"/>
<!-- first part must be castOn-->
<xs:assert id = "test-first-castOn" test = "if (/#partNumber= '0') then #type = 'castOnPartSeg' else false()"/>
</xs:complexType>
</xs:element>
<xs:element name="castOffPartPoint">
<xs:complexType>
<xs:sequence>
<xs:element ref ="basePoint"/>
</xs:sequence>
<xs:attribute name ="nextsnum" type = "kpRefsList" use = "required"/>
<xs:attribute name = "partNumber" type = "xs:nonNegativeInteger" use = "required"/>
</xs:complexType>
</xs:element>
<xs:element name="castOnPartSeg">
<xs:complexType>
<xs:sequence>
<xs:element ref ="baseSeg"/>
</xs:sequence>
<xs:attribute name = "nextsnum" type = "kpRefsList" use = "required"/>
<xs:attribute name = "partNumber" type = "xs:nonNegativeInteger" use = "required"/>
</xs:complexType>
</xs:element>
<xs:element name="joinPart">
<xs:complexType>
<xs:sequence>
<xs:element ref ="baseSeg"/>
</xs:sequence>
<xs:attribute name ="nextsnum" type = "kpRefsList" use = "required"/>
<xs:attribute name ="previousnum" type = "kpRefsList" use = "required"/>
<xs:attribute name = "partNumber" type = "xs:nonNegativeInteger" use = "required"/>
</xs:complexType>
</xs:element>
<xs:simpleType name = "kpRefsList">
<xs:list itemType= "xs:nonNegativeInteger"/>
</xs:simpleType>
<xs:element name = "basePoint">
<xs:complexType>
<xs:attribute name= "start" type = "point" use = "required"/>
</xs:complexType>
</xs:element>
<xs:element name = "baseSeg">
<xs:complexType>
<xs:attribute name= "start" type = "point" use = "required"/>
<xs:attribute name= "end" type = "point" use = "required"/>
<!--startY = endY!-->
<xs:assert id = "test_base_horizontal" test = "/#start[2] = /#end[2]"/>
</xs:complexType>
</xs:element>
<xs:simpleType name= "point">
<xs:restriction>
<xs:simpleType>
<xs:list itemType = "decimal5digits"/>
</xs:simpleType>
<xs:length value = "2"/>
</xs:restriction>
</xs:simpleType>
<xs:simpleType name ="decimal5digits">
<xs:restriction base = "xs:decimal">
<xs:fractionDigits value="5"/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
both assertions above always return false.
It should return true for this xml
<ns0:kPartModel xmlns:ns0="http://www.ludd21.com/kPartModel" modelName="manysingulars" otherattribute="">
<ns0:kPartsPiece pieceName="pieceknit_layer1">
<ns0:kPartsList>
<ns0:castOnPartSeg nextsnum="1" partNumber="0">
<ns0:baseSeg start="9.73143 0.00000" end="17.73188 0.00000" />
</ns0:castOnPartSeg>
<ns0:joinPart nextsnum="2" previousnum="0 2" partNumber="1" >
<ns0:baseSeg start="9.88173 -11.82912" end="25.27907 -11.82912" />
</ns0:joinPart>
<ns0:castOffPartPoint partNumber="2">
<ns0:basePoint start="21.83789 -8.02031"/>
</ns0:castOffPartPoint>
</ns0:kPartsList>
</ns0:kPartsPiece>
</ns0:kPartModel>