'Returns the extent of this geometry as a 4-tuple, consisting of
(xmin, ymin, xmax, ymax).'

@property
def extent(self):

    env = self.envelope
    if isinstance(env, Point):
        (xmin, ymin) = env.tuple
        (xmax, ymax) = (xmin, ymin)
    else:
        (xmin, ymin) = env[0][0]
        (xmax, ymax) = env[0][2]
    return (xmin, ymin, xmax, ymax)
