System information (version)

• OpenCV => opencv-python-headless 4.4.0.46
• Operating System / Platform => Ubuntu Linux, x64

Detailed description

Different from #18821 in cause, though the result is the same. In this case, the error I am causing is because the Python binding can not handle pathlib paths.

The real problem is that the C++ binding to Python does not properly set the error when "CV_Error" is caught.

I suspect the call stack looks like:

• Some python binding. Issue may be here

• opencv/modules/dnn/src/darknet/darknet_importer.cpp

  Line 184 in b866d0d

  static Net readNetFromDarknet(std::istream &cfgFile, std::istream &darknetModel)

• opencv/modules/dnn/src/darknet/darknet_importer.cpp

  Line 70 in b866d0d

  DarknetImporter(std::istream &cfgStream, std::istream &darknetModelStream)

• opencv/modules/dnn/src/darknet/darknet_io.cpp

  Line 1001 in b866d0d

  void ReadNetParamsFromCfgStreamOrDie(std::istream &ifile, darknet::NetParameter *net)

• Wherever CV_Error lives

Steps to reproduce

• Git clone darknet to a directory via git clone https://github.com/pjreddie/darknet.git
• Download YOLO weights from 'https://pjreddie.com/media/files/yolov3.weights'
• Run: pip install opencv-python-headless or pip install opencv-python

import cv2
from pathlib import Path
darknet_path = Path('<path to darknet src>')
yolo_cfg_path = darknet_path.joinpath('cfg/yolov3.cfg')
yolo_weights_path = Path('<path to downloaded weights>')
net = cv2.dnn.readNetFromDarknet(yolo_cfg_path, yolo_weights_path)

Should result in:

Traceback (most recent call last):
File "", line 1, in
SystemError: returned NULL without setting an error

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues,
  answers.opencv.org, Stack Overflow, etc and have not found solution
• I updated to latest OpenCV version and the issue is still there
• There is reproducer code and related data files: videos, images, onnx, etc