Question

I’m using lxml (2.2.8) to create and write out some XML (specifically XGMML). The app which will be reading it is apparently fairly fussy and wants to see a top level element with:

<graph label="Test" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:xlink="h
ttp://www.w3.org/1999/xlink" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-
ns#" xmlns:cy="http://www.cytoscape.org" xmlns="http://www.cs.rpi.edu/XGMML"  di
rected="1">

How do I setup those xmlns: attributes with lxml ? If I try the obvious

root.attrib['xmlns:dc']='http://purl.org/dc/elements/1.1/'
root.attrib['xmlns:xlink']='http://www.w3.org/1999/xlink'
root.attrib['xmlns:rdf']='http://www.w3.org/1999/02/22-rdf-syntax-ns#'
root.attrib['xmlns:cy']='http://www.cytoscape.org'
root.attrib['xmlns']='http://www.cs.rpi.edu/XGMML'

lxml throws a ValueError: Invalid attribute name u'xmlns:dc'

I’ve used XML and lxml a fair amount in the past for simple stuff, but managed to avoid needing to know anything about namespaces so far.

Answer 1

Unlike ElementTree or other serializers that would allow this, lxml needs you to set up these namespaces beforehand:

NSMAP = {"dc" : 'http://purl.org/dc/elements/1.1',
         "xlink" : 'http://www.w3.org/1999/xlink'}

root = Element("graph", nsmap = NSMAP)

(and so on and so forth for the rest of the declarations)

And then you can use the namespaces using their proper declarations:

n = SubElement(root, "{http://purl.org/dc/elements/1.1}foo")

Of course this gets annoying to type, so it is generally beneficial to assign the paths to short constant names:

DCNS = "http://purl.org/dc/elements/1.1"

And then use that variable in both the NSMAP and the SubElement declarations:

n = SubElement(root, "{%s}foo" % (DCNS))