[OE-core] confusion between += and =+ and _append and _prepend ...

[Martin Jansa]

On Sat, Mar 24, 2012 at 04:34:40AM -0400, Robert P. J. Day wrote:
> 
>   when perusing the source for oe-core, it's going to be easy for a
> beginner to get confused about the right time(s) to use += versus =+
> versus _append versus _prepend because the source simply bounces
> around between them all arbitrarily.
> 
>   here's a single example.  the file bitbake.conf defines the default
> for PACKAGES:
> 
> PACKAGES = "${PN}-dbg ${PN}-staticdev ${PN} ${PN}-doc ${PN}-dev ${PN}-locale"
> 
> now it would seem that, if a recipe wanted to extend that list (as
> many do), the obvious approach is to augment the list with more
> entries.  and in the occasional situation where that default needs to
> be overridden, then a straight assignment can be used.
> 
>   but searching the source shows every possible combination of adding
> to the PACKAGES variable:
> 
> ...
> meta/recipes-devtools/perl/perl_5.14.2.bb:PACKAGES_append = " perl-modules "
> meta/recipes-qt/qt4/qt-4.8.0.inc:PACKAGES_append = " ${QT_BASE_NAME}-tests-dbg ${QT_BASE_NAME}-tests"
> ...
> meta/recipes-connectivity/bind/bind_9.8.1.bb:PACKAGES_prepend = "${PN}-utils "
> meta/recipes-connectivity/wpa-supplicant/wpa-supplicant-0.7.inc:PACKAGES_prepend = "wpa-supplicant-passphrase wpa-supplicant-cli "
> meta/recipes-extended/newt/libnewt_0.52.14.bb:PACKAGES_prepend = "whiptail "
> ...
> 
>   and there are, of course, many, many uses of "PACKAGES +=" and
> "PACKAGES =+".  and, sure, they'll all work but the variety is
> definitely going to confuse someone trying to figure out best
> practices.
> 
>   and occasionally, there is a direct assignment when a simple
> appending would work fine, like with rpm:
> 
> meta/recipes-devtools/rpm/rpm_5.4.0.bb:PACKAGES = "${PN}-dbg ${PN}
> ${PN}-doc ${PN}-libs ${PN}-dev ${PN}-staticdev ${PN}-common
> ${PN}-build python-rpm-dbg python-rpm-staticdev python-rpm
> perl-module-rpm perl-module-rpm-dev ${PN}-locale"
> 
>   i realize this is all annoyingly picky pedantry but there should be
> some sort of standard for things like this, just for consistency.

order of items in PACKAGES variable is important (FILES_foo) are picked
in PACKAGES order so it's different if foo is before bar or after (if
their FILES_ mask has some overlap).

So sometimes you have to prepend, sometimes append.

Cheers,

-- 
Martin 'JaMa' Jansa     jabber: Martin.Jansa at gmail.com
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