Based on demo in https://tangopardo.com.ar/2cf7/2013/07/30/using-the-gstreamer-controller-subsystem-from-python

gst-demo.py

#!/usr/bin/python
import gi
import sys
from gi.repository import GObject
gi.require_version('Gst', '1.0')
from gi.repository import Gst
from gi.repository import GstController
from gi.repository import GLib

GObject.threads_init()
Gst.init(sys.argv)

p = Gst.parse_launch ("""
    videotestsrc pattern="colors" ! decodebin ! mix.sink_0
    videotestsrc pattern="black" ! decodebin ! mix.sink_1
    videomixer name="mix" sink_1::ypos=25 sink_1::xpos=25 sink_1::zorder=99 ! videoconvert ! videoscale ! autovideosink
""")

mix = p.get_by_name ("mix")
if not mix:
    raise Exception('Cannot find mix element')
for pad in mix.pads:
    print pad.name
#sink1 = [pad for pad in mix.pads if pad.name == 'sink_1'][0]
sink1 = mix.get_static_pad('sink_1')
if not sink1:
    raise Exception('Cannot find sink_1')

cs = GstController.InterpolationControlSource()
cs.set_property('mode', GstController.InterpolationMode.LINEAR)

cb = GstController.DirectControlBinding.new(sink1, 'alpha', cs)
sink1.add_control_binding(cb)

cs.set(0*Gst.SECOND, 1)
cs.set(4*Gst.SECOND, 0.5)

p.set_state (Gst.State.PLAYING)
GLib.MainLoop.new(None, False).run()