DNN LayerFactory

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Hi, I am using the dnn module of opencv 3.4.1 with Python 3.5 to deploy nets trained with keras/tensorflow. Some simple models can be deployed in python and c++ with opencv successfully. Now, I would like to implement the missing ClipByValue node/layer and register this layer in the dnn module with python. I am following this tutorial. Unfortuneatly I do not find the function dnn_registerLayer.

import cv2
cv2.dnn_registerLayer(...)

results in

AttributeError: module 'cv2.cv2' has no attribute 'dnn_registerLayer'

where do I find the module for register custom layers in Python?