Empty lists do not get serialized

[teobouvard]

Hi,

When I use the following schema

<?xml version="1.0" encoding="utf-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
    <xsd:element name="Wrapper" type="wrapper"/>

    <xsd:complexType name="wrapper">
        <xsd:sequence>
            <xsd:element name="ListOfNumbers" type="listOfNumbers" />
        </xsd:sequence>
    </xsd:complexType>
    
    <xsd:simpleType name="listOfNumbers">
        <xsd:list itemType="xsd:long" />
    </xsd:simpleType>
</xsd:schema>

to create a Wrapper object with an empty ListOfNumbers and I serialize it using the code below

from xsdata.formats.dataclass.serializers import XmlSerializer
from xsdata.formats.dataclass.serializers.config import SerializerConfig

from generated.schema import Wrapper

obj = Wrapper()
obj.list_of_numbers = []

config = SerializerConfig(pretty_print=True)
serializer = XmlSerializer(config)

print(serializer.render(obj))

The ListOfNumbers element do not get serialized which results in the following XML file.

<?xml version="1.0" encoding="UTF-8"?>
<Wrapper/>

First of all, is this the expected behaviour ? I find it surprising to not have an empty element in the generated XML, which I expected to look somewhat like this

<?xml version="1.0" encoding="UTF-8"?>
<Wrapper>
    <ListOfNumbers/>
</Wrapper>

If this is the expected behaviour, would you be open to a pull request adding an option to serialize empty lists as empty elements rather than discard them?

[teobouvard]

I tried reading the generated XML with the following sample, and it works.

from xsdata.formats.dataclass.parsers import XmlParser
from xsdata.formats.dataclass.context import XmlContext

from generated.schema import Wrapper

parser = XmlParser(context=XmlContext())
wrapper = parser.parse("sample.xml", Wrapper)

The issue is that the generated XML is not compliant with the provided schema.

$ xmllint --noout --schema schema.xsd sample.xml 
sample.xml:2: element Wrapper: Schemas validity error : Element 'Wrapper': Missing child element(s). Expected is ( ListOfNumbers ).
sample.xml fails to validate