error importing cv2 in python3.6 but it works for python2.7

asked 2020-08-08 11:25:30 -0500

PKGosal gravatar image

updated 2020-08-08 19:02:01 -0500

Error I get when using python3.6

>>> import cv2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: /usr/local/lib/python3.6/dist-packages/cv2.so: undefined symbol: PyCObject_Type

However, when I use python2.7, I get no errors

>>> import cv2
>>> cv2.__version__
'4.0.0'

I installed opencv 4.0.0 from source and ensured proper bindings to Python2.7 and Python3.6. Ensured that ..../dist-packages for both versions of python had cv2.so file. See contents of dis-packages below.

$ls /usr/local/lib/python2.7/dist-packages
cv2.so
$ ls /usr/local/lib/python3.6/dist-packages
clonevirtualenv.py                  pip-20.2.1.dist-info
cv2.so                              py
decorator-4.4.0.dist-info           py-1.8.0.dist-info
decorator.py                        __pycache__
future                              pykalman
future-0.17.1.dist-info             pykalman-0.9.5.dist-info
importlib_metadata                  stevedore
importlib_metadata-1.7.0.dist-info  stevedore-3.2.0.dist-info
libfuturize                         virtualenv-16.6.0.dist-info
libpasteurize                       virtualenv_clone-0.5.4.dist-info
networkx                            virtualenv.py
networkx-2.3.dist-info              virtualenv_support
numpy                               virtualenvwrapper
numpy-1.19.1.dist-info              virtualenvwrapper-4.8.4.dist-info
numpy.libs                          virtualenvwrapper-4.8.4-nspkg.pth
past                                zipp-3.1.0.dist-info
pip                                 zipp.py

SO, I see that cv2.so file is there in both dist-packages... I am not sure why I am seeing the error for Python3.6. PS: The cv2.so file was initially missing from dis-packages of python3.6 when I checked after successful completion of cmake and make... So, I manually copied the cv2.so file from dist-packages of python2.7 and pasted it in dist-packages of oython3.6.. (I am not sure if this was the right thing to do!) SO, the file cv2.so is essentially the same in both versions of python.

Below is how i built from source.

cmake -D CMAKE_BUILD_TYPE=RELEASE \
-D CMAKE_INSTALL_PREFIX=/usr/local \
-D INSTALL_PYTHON_EXAMPLES=ON \
-D INSTALL_C_EXAMPLES=OFF \
-D OPENCV_ENABLE_NONFREE=ON \
-D OPENCV_EXTRA_MODULES_PATH=~/opencv_contrib/modules \
-D PYTHON3_EXECUTABLE=$(which python3.6) \
-D PYTHON2_EXECUTABLE=$(which python2.7) \
-D PYTHON2_INCLUDE_DIR=$(python2.7 -c "from distutils.sysconfig import get_python_inc; print(get_python_inc())") \
-D PYTHON2_INCLUDE_DIR2=/usr/include/aarch64-linux-gnu/python2.7 \
-D PYTHON3_INCLUDE_DIR=$(python3.6 -c "from distutils.sysconfig import get_python_inc; print(get_python_inc())") \
-D PYTHON3_INCLUDE_DIR2=/usr/include/aarch64-linux-gnu/python3.6m \
-D PYTHON2_LIBRARY=/usr/lib/python2.7/config-aarch64-linux-gnu/libpython2.7.so \
-D PYTHON3_LIBRARY=/usr/lib/python3.6/config-3.6m-aarch64-linux-gnu/libpython3.6.so \
-D PYTHON2_NUMPY_INCLUDE_DIR=$(python2.7 -c "import numpy; print(numpy.get_include())") \
-D PYTHON3_NUMPY_INCLUDE_DIR=$(python3 -c "import numpy; print(numpy.get_include())") \
-D INSTALL_PYTHON_EXAMPLES=ON \
-D BUILD_opencv_python3=ON \
-D BUILD_opencv_python2=ON \
-D HAVE_opencv_python2=ON \
-D HAVE_opencv_python3=ON \
-D PYTHON_DEFAULT_EXECUTABLE=$(which python2.7) \
-D BUILD_NEW_PYTHON_SUPPORT=ON \
-D BUILD_PYTHON_SUPPORT=ON \
-D OPENCV_PYTHON3_INSTALL_PATH=/usr/src/app \
-D BUILD_TESTS=OFF \
-D BUILD_PERF_TESTS=OFF \
-D PYTHON3_CVPY_SUFFIX=.cpython-36m-aarch64-linux-gnu.so \
-D WITH_CUDA=OFF \
-D WITH_TBB=ON \
-D OPENCV_SKIP_PYTHON_LOADER=ON \
-D BUILD_opencv_hdf=OFF \
-D BUILD_EXAMPLES=ON ..

This is part of the output on the console after cmake command.

-- General configuration for OpenCV 4.0.0 =====================================

-- Version control: unknown

-- Extra modules: -- Location (extra): /home/prabhjot/opencv_contrib/modules

-- Version control (extra): unknown

-- Platform: -- Timestamp: 2020-08-08T23:55:46Z -- Host: Linux 5.4.0-42-generic x86_64 -- CMake: 3.10.2 ... (more)

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Comments

I installed opencv 4.0.0 from source

unfortunately, that's far too old (probably noone will be able to reproduce your problems), we're at 4.4 currently

berak gravatar imageberak ( 2020-08-08 11:49:53 -0500 )edit

however, please add the cmake (console) output, maybe it has some info, why (or if) py3 was dropped

berak gravatar imageberak ( 2020-08-08 11:50:43 -0500 )edit

So, I manually copied the cv2.so

no, you should run make install instead

berak gravatar imageberak ( 2020-08-08 11:51:46 -0500 )edit

@berak Thanks for the quick response. I did a clean re-install by running (sudo cmake uninstall) ffirst rom my build directory and confirmed that previous built files were all removed. I also removed cv2.so files from dist-package/site-package... Deleted the built directory and did a clean Reinstall...... I used the same Cmake variables that I used before (shown above). I have added the output of Cmake to my original question above . I also did sudo make install after make -j7.. But I am getting the same results as before. Note that cv2.so files were missing from site-packges and dist-packages for both python2.7 and python3.6 after all of this. But they were present in /usr.local/python/python2.7 and /usr/local/python/python3.6... So, I am still getting the same result as mentioned above

PKGosal gravatar imagePKGosal ( 2020-08-08 19:33:11 -0500 )edit

Is it possible that because I have -D PYTHON_DEFAULT_EXECUTABLE=$(which python2.7) \ in Cmake that may be leading to this end result? Note that because cv2.so files were not present in site-packages/ dist-packages ... I manually added them for both python2 and python3. ANd the end result is that cv2 is successfully imported in python2 but not in python3.

PKGosal gravatar imagePKGosal ( 2020-08-08 19:34:51 -0500 )edit

Python 2 is obsoleted. But why you want that?

supra56 gravatar imagesupra56 ( 2020-08-09 20:52:25 -0500 )edit