This question is related to: Python SOAP server / client
In the case of soap with python, there are recommendation to use soaplib (http://wiki.github.com/jkp/soaplib
try to import primitives into your class:
class HelloWorldService(SimpleWSGISoapApp):
from soaplib.serializers.primitive import String, Integer, Arraycode
@soapmethod(String,Integer,_returns=Array(String))
this bug is fixed if you get the latest sources from the trunk, see https://github.com/soaplib/soaplib/pull/12 for details