Using OpenCV imread and imwrite with Python Path objects gives SystemError: <built-in function imread> returned NULL without setting an error

Question

Passing a Python Path object to OpenCV's imread or imwrite results in an undefined error:

from pathlib import Path
import cv2

img_path = Path("test.png")
img = cv2.imread(img_path)

Results in:

Traceback (most recent call last):
  File ".\secondary_image_generation.py", line 36, in <module>
    img = cv2.imread(img_path)
SystemError: <built-in function imread> returned NULL without setting an error

Why is this and how can I avoid it?

Answer 1

OpenCV library sources are written in C++ and the Python bindings are primarily auto-generated and don't do much more than wrap the C++ functions. The C++ functions expect string type filenames so that is what you have to provide to the Python functions as well.

Doing the following resolves the problem by resolving the full path and converting it to a string:

img_path = Path("test.png")
img = cv2.imread(str(img_path.resolve()))

This is still an open feature request for the current version of OpenCV.