Gentoo Development Guide

The Portage Cache

Portage uses a cache for most top-level variables (DEPEND, DESCRIPTION, SRC_URI and so on). This cache may be generated on a different machine, so these variables must be either static or generated using only unchanging 'version / name' variables (P, PN, PV, PR, PVR and PF).

So, the following will not work:

# DO NOT DO THIS!
if ! has_version "x11-libs/gtk+" ; then
    DEPEND="${DEPEND}
            gtk?  ( >=x11-libs/gtk+-2 )
            !gtk? ( =x11-libs/gtk+-1.2* )"
fi

However, this is legal, since versionator.eclass works upon PV, and PV and PN are both static:

inherit versionator

if [[ $(get_major_version) -ge 7 ]] ; then
    IUSE="${IUSE} tcltk mzscheme"
    DEPEND="${DEPEND}
        tcltk?    ( dev-lang/tcl )
        mzscheme? ( dev-lisp/mzscheme )"
    RDEPEND="${RDEPEND}
        tcltk?    ( dev-lang/tcl )
        mzscheme? ( dev-lisp/mzscheme )"

    if [[ "${MY_PN}" != "vim-core" ]] ; then
        RDEPEND="${RDEPEND} !<app-vim/align-30-r1"
    fi
fi

Conditional Inherits

Because eclasses modify various cached variables, conditional inheritance is not allowed except where the same results will always be obtained on every system. For example, inherits based upon USE flags are illegal, but inherits based solely upon PN are allowed.

As an example of a legal and possibly useful conditional inherit, some eclasses do:

if [[ "${PN##*-}" == "cvs" ]] ; then
    inherit cvs
fi

This allows the same eclass to be used for both regular and -cvs packages.