REQUIRE_USE for single_python_target would be helpful as it detects the problems during calculation phase instead of merge.
+ 02 May 2014; Pacho Ramos <pacho@gentoo.org> glade-3.16.1.ebuild, + glade-3.18.2.ebuild, glade-3.8.4.ebuild: + Set PYTHON_REQUIRED_USE (#509318 by Maciej Piechotka) +