You may use a custom calculation callback, instead of a standard clutter.AnimationMode enum value such as LINEAR, by using the actor.animate_with_alpha() function, providing a clutter.Alpha.

The clutter.Alpha object is constructed with a calculation callback and a clutter.Timeline which tells it when a new frame needs a new value to be calculated. Your alpha callback will need to call alpha.get_timeline() so it can return a value based on the timeline's current progress, using the timeline.get_progress() function.

See the Behaviours chapter for an example that uses a clutter.Alpha.