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> telepathy-idle-0.1.16.ebuild, + telepathy-idle-0.2.0.ebuild: + Set PYTHON_REQUIRED_USE (#509316 by Maciej Piechotka) +