def factory():
    document = {'root': None,
                'stack': []}

    class Tag(object):

        def __init__(self, name, namespace='', **attrs):

            _base = namespace + name if namespace else name

            self._el = etree.Element(_base, **attrs)

        def __enter__(self):
            if document['root'] is None:
                document['root'] = self._el
            document['stack'].append(self._el)
            return self._el

        def __exit__(self, *args):
            document['stack'].pop()
            if document['stack']:
                document['stack'][-1].append(self._el)

    return Tag, document


tag, doc = factory()

with tag('GetRequestRequest', NS_1):
    with tag('MessageTypeSelector', NS_2):
        with tag('NamespaceURI') as ns_uri:
            ns_uri.text = 'some bullshit about ns'
        with tag('RootElementLocalName'):
            pass
        with tag('Timestamp'):
            pass
    with tag('CallerInformationSystemSignature', NS_1):
        with tag('Signature'):
            pass