So, the signature should contain the special argument while a calltip probably should omit it as you must not pass it when calling the method. On the other hand, this would require even more Python specific code :(.
Pyeany? Oh wait its not April 1 ;-P
Agree with @techee that sprinkling language specific code throughout is nasty, it makes using things like LSPs harder. But anyway it should be a separate PR, and maybe it could start a "language_specific.c" file and put it in a function there, and other language specific parts of Geany could be migrated over (a long) time.
But on this PR, the last example replacing the first and the others the same looks great to me.