$ cat meta-local/recipes-example/example/example_0.1.bb
SUMMARY = "bitbake-layers recipe"
DESCRIPTION = "Recipe created by bitbake-layers"
LICENSE = "MIT"

python do_build() {
    bb.plain("***********************************************");
    bb.plain("*                                             *");
    bb.plain("*  Example recipe created by bitbake-layers   *");
    bb.plain("*                                             *");
    bb.plain("***********************************************");
}

python do_foobar_prepend() {
    d.setVar("FOO", "well this should work")
}

python do_foobar () {
    print("FOO is set to: " + d.getVar("FOO"))
}

addtask foobar after do_fetch before do_build


$ cat tmp/work/cortexa7t2hf-neon-vfpv4-poky-linux-gnueabi/example/0.1-r0/temp/log.do_foobar
DEBUG: Executing python function do_foobar
DEBUG: Python function do_foobar finished
FOO is set to: well this should work