test_extra_args.py

from gi.repository import BlockDev as blockdev

blockdev.init()

def print_args(*args):
    print(args)

# set print_args() as the logging function
blockdev.reinit(None, False, print_args)

if __name__ == "__main__":
    extra = [blockdev.UtilExtraArg.new(opt, val) for opt, val in (("-L", "TEST_LABEL"), ("-v", ""))]
    blockdev.fs_ext4_mkfs("/dev/loop0", extra)

Of course, it's easy to transform (_args, *_kwargs) into a list of blockdev.UtilExtraArg objects which is what we can/will do in the Python GI overrides. So the function will just look like this: fs_ext4_mkfs(device, *args, **kwargs) with all the magic happening automatically.