Array of subclasses of a type xml output issue

[Omar]

Hello,

When I run

$ pyxbgen -u xsd -m mynf
$ python test.py

the output is

<ns1:B xmlns:ns1="mynf"/> <ns1:ArrayOfA xmlns:ns1="mynf"><ns1:A/></ns1:ArrayOfA>

I would expect the array to be:
<?xml version="1.0" ?><ns1:ArrayOfA xmlns:ns1="mynf"><ns1:B/></ns1:ArrayOfA>

Is this a bug, or am I missing something?

test.py is

import mynf
array = mynf.ArrayOfA()
b = mynf.B()
array.append(b)
print b.toxml()
print array.toxml()

test.xsd

<?xml version="1.0" encoding="utf-8"?>
<xs:schema targetNamespace="mynf" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:tns="mynf" elementFormDefault="qualified">

<xs:complexType name="ArrayOfAType">
<xs:sequence><xs:element minOccurs="0" maxOccurs="unbounded" name="A" nillable="true" type="tns:AType"/></xs:sequence>
</xs:complexType>

<xs:element name="ArrayOfA" nillable="true" type="tns:ArrayOfAType"/>

<xs:complexType name="AType"/>
<xs:element name="A" nillable="true" type="tns:AType"/>

<xs:complexType name="BType"><xs:complexContent mixed="false">
    <xs:extension base="tns:AType"/></xs:complexContent>
</xs:complexType>
<xs:element name="B" nillable="true" type="tns:BType"/>

</xs:schema>

[Peter A. Bigot]

No, PyXB is actually correct here, as demonstrated by independent validation:

llc[106]$ xmllint --schema x.xsd xa.xml 
<?xml version="1.0"?>
<ns1:ArrayOfA xmlns:ns1="mynf"><ns1:A/></ns1:ArrayOfA>
xa.xml validates
llc[107]$ xmllint --schema x.xsd xb.xml 
<?xml version="1.0"?>
<ns1:ArrayOfA xmlns:ns1="mynf"><ns1:B/></ns1:ArrayOfA>
xb.xml:1: element B: Schemas validity error : Element '{mynf}B': This element is not expected. Expected is ( {mynf}A ).
xb.xml fails to validate

In short, your schema does not allow an element {mynf}B to occur within complex type {mynf}ArrayOfAType. PyXB is being helpful here, and implicitly converting the (empty) content of the instance you pass into an instance A which is permitted.

There are two underlying issues. First, you have two distinct elements A: a global {mynf}A and a local A with no namespace{mynf}A that is scoped only within {mynf}ArrayOfAType. This actually is perfectly valid, but is probably not what you intend.

The second is that XML is not like object-oriented languages where B extends A means B isa A. You need to use substitution groups to accomplish what you want. Here's a schema that works:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema targetNamespace="mynf" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:tns="mynf" elementFormDefault="qualified">

  <xs:complexType name="ArrayOfAType">
    <xs:sequence>
      <!-- Change this to be a reference to {mynf}A -->
      <xs:element ref="tns:A" minOccurs="0" maxOccurs="unbounded"/>
    </xs:sequence>
  </xs:complexType>

  <xs:element name="ArrayOfA" nillable="true" type="tns:ArrayOfAType"/>

  <xs:complexType name="AType"/>
  <xs:element name="A" nillable="true" type="tns:AType"/>

  <xs:complexType name="BType">
    <xs:complexContent mixed="false">
      <xs:extension base="tns:AType"/>
    </xs:complexContent>
  </xs:complexType>
  <!-- Add this to the substitution group for {mynf}A -->
  <xs:element name="B" nillable="true" type="tns:BType" substitutionGroup="tns:A"/>

</xs:schema>

Then your test program produces the desired output (and both sample documents validate).