foobar2.py

import bpy


class SimpleCallback(bpy.types.Operator):
    bl_idname = "object.variable_callback"
    bl_label = "Simple Callback"

    variable_name = bpy.props.StringProperty()
    ops_kind = bpy.props.FloatProperty()

    def execute(self, context):
        
        if self.ops_kind in {2.0, 0.5}:
            scn = context.scene
            current_value = getattr(scn, self.variable_name)
            setattr(scn, self.variable_name, int(current_value * self.ops_kind))

        return {'FINISHED'}


class HelloVariableOperator(bpy.types.Operator):
    """Creates a Panel in the Object properties window"""
    bl_idname = "object.variable_operator"
    bl_label = "Simple Variable Operator"
    bl_options = {'REGISTER', 'UNDO'}
        
    def draw(self, context):
        layout = self.layout
        row = layout.row(align=True)
        
        # left
        op = row.operator("object.variable_callback", icon="ZOOMOUT", text="")
        op.ops_kind = 0.5
        op.variable_name = 'some_value'
        
        # middle
        scn = bpy.context.scene
        print(scn.some_value)
        row.prop(scn, "some_value", text='some_value')
        
        # right
        op = row.operator("object.variable_callback", icon="ZOOMIN", text="")
        op.ops_kind = 2.0
        op.variable_name = 'some_value'

    def execute(self, context):
        return {'FINISHED'}        


def register():
    bpy.utils.register_class(SimpleCallback)
    bpy.utils.register_class(HelloVariableOperator)
    bpy.types.Scene.some_value = bpy.props.IntProperty(default=30)


def unregister():
    del bpy.types.Scene.some_value
    bpy.utils.unregister_class(HelloVariableOperator)
    bpy.utils.unregister_class(SimpleCallback)
    


if __name__ == "__main__":
    register()