find xml element based on its attribute and change its value

I am using python xmlElementTree and want to assign or modify a xml element value based on its attribute. Can somebody give me an idea how to do this?

For example: Here is a xml file and I need to set the value for the element "number" based on the attribute "sys/phoneNumber/1", "sys2/SMSnumber/1" and so on.

<root>
    <phoneNumbers>
        <number topic="sys/phoneNumber/1" update="none" />
        <number topic="sys/phoneNumber/2" update="none" />
        <number topic="sys/phoneNumber/3" update="none" />
    </phoneNumbers>

    <gfenSMSnumbers>
        <number topic="sys2/SMSnumber/1" update="none" />
        <number topic="sys2/SMSnumber/2" update="none" />
    </gfenSMSnumbers>
</root>

edit: Added closure for the tag root in the XML file.