Howto send a valid XML POST request to ebay API via Python?

Question

I guess this is rather a general problem than ebay specific, but I am not sure: I am trying to send an XML request to the ebay developer API to retrieve an XML response. When using curl, everything works fine and I get an XML response telling me which API-keys are missing (if I would provide them via HTTP headers I would get a valid XML result):

curl -d '<!xml version="1.0" encoding="utf-8"?><findItemsByKeywordsRequest xmlns="http://www.ebay.com/marketplace/search/v1/services"><keywords>harry potter phoenix</keywords></findItemsByKeywordsRequest>' \
http://svcs.sandbox.ebay.com/services/search/FindingService/v1

Which leads to the correct response:

<?xml version='1.0' encoding='UTF-8'?>
<ms:errorMessage xmlns:ms="http://www.ebay.com/marketplace/services" xmlns="http://www.ebay.com/marketplace/search/v1/services">
    <error>
        <errorId>2038</errorId>
        <domain>CoreRuntime</domain>
        <severity>Error</severity>
        <category>System</category>
        <message>Missing SOA operation name header</message>
        <subdomain>System</subdomain>
    </error>
</ms:errorMessage>

But when I try to work with Python I just get "500 Internal Server error", no matter how basic I make my examples. I have tried two very basic methods:

Number one:

serverUrl = 'svcs.sandbox.ebay.com'
xmlparameters = '<!xml version="1.0" encoding="utf-8"?><findItemsByKeywordsRequest xmlns="http://www.ebay.com/marketplace/search/v1/services"><keywords>harry potter phoenix</keywords></findItemsByKeywordsRequest>'

webservice = httplib.HTTP(serverUrl)
webservice.putrequest("POST", "/services/search/FindingService/v1")
webservice.putheader("Host", serverUrl)
webservice.putheader("Content-type", "text/xml; charset=\"UTF-8\"")
webservice.putheader("Content-length", "%d" % len(xmlparameters))
webservice.endheaders()
webservice.send(xmlparameters)

Number two (which is my prefered method):

serverUrl = 'svcs.sandbox.ebay.com'
xmlparameters = '<!xml version="1.0" encoding="utf-8"?><findItemsByKeywordsRequest xmlns="http://www.ebay.com/marketplace/search/v1/services"><keywords>harry potter phoenix</keywords></findItemsByKeywordsRequest>'

connection = httplib.HTTPConnection(serverUrl)
connection.request("POST", '/services/search/FindingService/v1', xmlparameters)

As you can see in the CURL example, it does not matter that I do not send the API keys etc., it should return an XML error response anyway and not only HTTP status code "500 Internal server error".

Does anyone see what I am doing wrong with my POST request?

[EDIT] btw using the URL ValueName API works perfectly with Python, but that’s just a GET request on a URL. Yet, I’d prefer to use the XML API. However, if that’s not possible I’d switch to ValueName URIs of course.