I wasn't sure what you were asking. I figured it was a code parameter for Python you were seeking. However, manually, within GIMP, there are ways to accomplish simple diamonds using Paths.
Take a look at the Shape Paths script for making polygon shapes:
solved by workaround
drawing a diamond and use fuzzy select. That's it
Are you using Paths?
I wasn't sure what you were asking. I figured it was a code parameter for Python you were seeking. However, manually, within GIMP, there are ways to accomplish simple diamonds using Paths.
Take a look at the Shape Paths script for making polygon shapes:
http://registry.gimp.org/node/59