xml = """<root xmlns="default_ns">
    <subelement>
        <!-- here we can have an element of any namespace  -->
        <some_prefix:a xmlns:some_prefix="some_namespace">
            <some_prefix:b/>
        </some_prefix:a>
    </subelement>
</root>"""
root = etree.fromstring(xml)
Нежелательное объявление пространства имен в lxml XPath