Which camera, and what functions are you using?
If you want to do the equivalent of setting manual ISO in the canon UI, you can set propcase ISO_MODE to the specific UI value (e.g. 100, 200). ISO_MODE should be set before half press, and will remain in effect until you change ISO in the Canon UI, use CHDK ISO overrides, reboot (I think) etc.
Setting values that are not accessible in the UI may not have the desired results, so for example on G7x, you should not set 100, since the minimum is 125, and you should not set 234 because that's not one of the standard steps.
To set an arbitrary ISO, you should use the set_sv96 function. This takes an APEX96 "real" value, so to set the equivalent of a UI ISO, you should do something like
set_sv96(sv96_market_to_real(iso_to_sv96(125)))
This takes effect only for the next shot, *but* the camera will effectively left in AUTO ISO for subsequent shots. You can save/restore propcase ISO_MODE before/after using it if you want to preserve whatever the previous state was.
set_sv96 can generally be used outside or inside half press though the latter may have issues in some cases.
edit:
Note that while you can set values outside the canon UI range with set_sv96, they will like not give you significantly expanded ISO range, and might crash.
