misleading error when libpython is missing

[cen-kalakaua]

📚 The doc issue

On debian 13 after installing torchcodec and ffmpeg (via apt-get install) importing torchcodec fails with "Could not load library: ... libtorchcodec_custom_ops7.so" (ops not core!). I straced it and what's actually missing is "apt-get install python3-dev" which install libpython3.13 along with it

[zzaudio]

Life saving information... Worked for my Ubuntu 24.04 + Python 3.12 + torch 2.9.1 + cuda 12.8 setting.

[lamikr]

Normally there should not be a need to link the libpython library directly as python interpreter can take of loading the symbols needed from it. So we could take a away the direct dependency to libpython from the libtorchcodec_custom_ops[4,5,6,7,8].so libraries.

Originally when I build the torchcodec and I checked the dependencies, these were required:

ldd .venv/lib/python3.12/site-packages/torchcodec/libtorchcodec_custom_ops8.so 
	linux-vdso.so.1 (0x00007f34822cc000)
	libtorchcodec_core8.so => not found
	libpython3.12.so.1.0 => /lib/x86_64-linux-gnu/libpython3.12.so.1.0 (0x00007f3481800000)
	libavutil.so.60 => not found
	libavcodec.so.62 => not found
	libavformat.so.62 => not found
	libavdevice.so.62 => not found
	libavfilter.so.11 => not found
	libswscale.so.9 => not found
	libswresample.so.6 => not found
	libtorch.so => not found
	libtorch_cpu.so => not found
	libtorch_hip.so => not found
	libc10.so => not found
	libstdc++.so.6 => /lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f3481400000)
	libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f3482227000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f3481000000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f348213c000)
	libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007f3482120000)
	libexpat.so.1 => /lib/x86_64-linux-gnu/libexpat.so.1 (0x00007f34820f4000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f34822ce000)

And if I removed the libpython3.12.so.1.0, I got also a same error if I executed tests which tried to import torchcodec. (pytest test/test_ops.py for example)

But I builded the torchcodec by removing the direct libpython linking dependency and then tested the the execution of torchcodec on python env when the libpython was not available, then torchcodec worked as expected for example when running the test_ops.py or other torchcodec tests.

These were the link dependencies I had in that case on the modified torchcodec build.

ldd external-builds/pytorch/pytorch_torchcodec/build/lib.linux-x86_64-cpython-312/torchcodec/libtorchcodec_custom_ops8.so
        linux-vdso.so.1 (0x00007fbfb477d000)
        libtorchcodec_core8.so => not found
        libavutil.so.60 => not found
        libavcodec.so.62 => not found
        libavformat.so.62 => not found
        libavdevice.so.62 => not found
        libavfilter.so.11 => not found
        libswscale.so.9 => not found
        libswresample.so.6 => not found
        libtorch.so => not found
        libtorch_cpu.so => not found
        libtorch_hip.so => not found
        libc10.so => not found
        libstdc++.so.6 => /lib64/libstdc++.so.6 (0x00007fbfb406b000)
        libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007fbfb3e53000)
        libc.so.6 => /lib64/libc.so.6 (0x00007fbfb3a7d000)
        libm.so.6 => /lib64/libm.so.6 (0x00007fbfb36fb000)
        /lib64/ld-linux-x86-64.so.2 (0x00007fbfb4400000)

I can submit a patch to remove this dependency. Maybe it would also be a good to change the original error message to:

"Could not load library: ... libtorchcodec_custom_ops7.so" or some of it's other library dependencies. (ops not core!).