def link_callback(uri, rel):
    """
    Convert HTML URIs to absolute system paths so xhtml2pdf can access those
    resources
    """
    # use short variable names
    sUrl = settings.STATIC_URL      # Typically /static/
    sRoot = settings.STATIC_ROOT    # Typically /home/userX/project_static/
    mUrl = settings.MEDIA_URL       # Typically /static/media/
    mRoot = settings.MEDIA_ROOT     # Typically /home/userX/project_static/media/

    # convert URIs to absolute system paths
    if uri.startswith(mUrl):
        path = os.path.join(mRoot, uri.replace(mUrl, ""))
    elif uri.startswith(sUrl):
        path = os.path.join(sRoot, uri.replace(sUrl, ""))
    else:
        return uri  # handle absolute uri (ie: http://some.tld/foo.png)

    # make sure that file exists
    if not os.path.isfile(path):
            raise Exception(
                'media URI must start with %s or %s' % (sUrl, mUrl)
            )
    return path