FloatProperty Slider Pairs That Sum to 1, Dynamically Changing

Question

This might just not be possible but I thought I'd ask. Is it possible to have two sliders min=0.0, max=1.0. Where their contents always sum to 1 but the user can still move them.

For example a slider A and slider B. When A is increased, B decreases by some linearly proportional amount.

I tried a couple different approaches, first by just setting the value in draw:

total = myPropGroup.probA + myPropGroup.probB
if total > 1.0:
   myPropGroup.probA = myPropGroup.probA / total
   myPropGroup.probB = myPropGroup.probB / total
layoutBox.prop(myPropGroup, "probA", text="Probability A", slider=True)
layoutBox.prop(myPropGroup, "probB", text="Probability B", slider=True)

Didn't even load.

I also tried using the set and update functions of the properties but caused some infinite loops.

Answer 1

Rough example.

Honestly a single variable could be used as a property and just subtract the value from 1 for any comparison.

import bpy


def upd_left(self, context):
    if not self.my_prop_right + self.my_prop_left == 1:
        self.my_prop_right = 1 - self.my_prop_left


def upd_right(self, context):
    if not self.my_prop_right + self.my_prop_left == 1:
        self.my_prop_left = 1 - self.my_prop_right


class TEST_PG_props(bpy.types.PropertyGroup):
    my_prop_left: bpy.props.FloatProperty(
        name="my_prop_left",
        default=0,
        min=0,
        max=1,
        update=upd_left,
        )

    my_prop_right: bpy.props.FloatProperty(
        name="my_prop_right",
        default=0,
        min=0,
        max=1,
        update=upd_right,
        )


class TEST_PT_test_panel(bpy.types.Panel):
    bl_idname = "TEST_PT_test_panel"
    bl_label = "Test Panel"
    bl_space_type = "VIEW_3D"
    bl_region_type = "UI"
    bl_category = "Test Tab"
 
    def draw(self, context):
        my_props = context.scene.my_props
        layout = self.layout
        row = layout.row()
        row.prop(my_props, "my_prop_left", text="left")
        row.prop(my_props, "my_prop_right", text="right")


classes = [
    TEST_PG_props,
    TEST_PT_test_panel,
    ]


def register():
    for cls in classes:
        bpy.utils.register_class(cls)
    bpy.types.Scene.my_props = bpy.props.PointerProperty(
            type=TEST_PG_props)


def unregister():
    for cls in classes:
        bpy.utils.unregister_class(cls)
    del bpy.types.Scene.my_props

if __name__ == "__main__":
    register()
```