def METHOD_NAME(mag):
    newmag = mag.get_value()
    oldmag = config.getfloat("browsers", "covergrid_magnification", 3.)
    if newmag == oldmag:
        print_d("Covergrid magnification haven't changed: {0}"
                .format(newmag))
        return
    print_d('Covergrid magnification update from {0} to {1}'
            .format(oldmag, newmag))
    config.set("browsers", "covergrid_magnification", mag.get_value())
    browser.update_mag()