Pyclip is a very simple, but flexible video editing library written in Python3.
import pyclip
nat = pyclip.Clip("res/nature.mp4")
dol = pyclip.Clip("res/dolphins.mov")
view = pyclip.Clip("res/view.mp4")
W, H = 1080, 720
view.info.position_type = "top-left"
nat.info.position_type = "center"
dol.info.position_type = "bottom-right"
dol.info.trans.scale.w = 0.5
dol.info.trans.scale.h = 0.5
nat.info.trans.scale.w = 0.4
nat.info.trans.scale.h = 0.4
nat.info.trans.rot.angle = 45
view.info.trans.scale.w = 0.2
view.info.trans.scale.h = 0.5
view.info.trans.rot.angle = 90
movie = pyclip.Movie("Name", W, H, 60)
movie.put_clip(nat, 0)
movie.put_clip(dol, 150)
movie.put_clip(view, 0)
# play or export movie
pyclip.play(movie)
pyclip.export(movie)