Add namespace and attribute to a XML

I have a xml file and I need to add a namespace and an attribute associated to the namespace.

The original xml looks this way

<ns0:quakeml xmlns:ns0="http://ift.tt/11LkHVW" xmlns="http://ift.tt/1uKZhBO">
<eventParameters publicID="smi:URI/id">
<event publicID="URI/id">

I need to add a namespace defined as

xmlns:catalog="http://ift.tt/11LkHW1"

and a parameter to the event tag'http://ift.tt/11LkHW1'

catalog:datasource="foo"

Actually I do :

xml.set("{http://ift.tt/11LkHW5", 'http://ift.tt/11LkHW1')

so I get

<ns0:quakeml xmlns:ns0="http://ift.tt/11LkHVW" xmlns="http://ift.tt/1uKZhBO" xmlns:ns1="http://ift.tt/11LkHW1" ns1:catalog="http://ift.tt/11LkHW1">

not exactly the same I want.

To insert the attribute I do

event.attrib['eventsource'] = 'foo'

but it doesnt add any namespace