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Well, there wasn't any indication before that the returned object was implementing the "addinfourl" interface. So I don't think we have lost anything. In what situation this interface is useful ? The following comment (that you had highlighted in your comment) gives the impression that theses methods are there only to provide compatibility with clients using old-styles responses. 
http://hg.python.org/cpython/file/c499cc2c4a06/Lib/http/client.py#l772
That would imply that newer clients would usually not use these methods. If you want to document this, I think the "addinfourl" interface should then be better described somewhere else where it would include the fact that the HTTPResponse class implements it.
Anyway, I don't see the advantage of using a getter method (like geturl()) instead of accessing directly the attribute. For me, this is less pythonic. If you ever have to attach a behaviour to the access of this attribute, a property could then be defined.