# mycustom.bbclass
python do_foo () {
        package = d.getVar('PN', True)
        version = d.getVar('PV', True)
        s = "$ \"{0}\",\"{1}\"\n".format(package,version)
        with open("foo.txt", "a") as logFile:
                logFile.write(s);
}
do_install[postfuncs] += "do_foo"