How do I write to a v4l2loopback device?

Created by: kno10

ffmpeg can write to v4l2loopback devices, e.g., ffmpeg -y -f bmp_pipe -i - -f v4l2 /dev/video4

But when I try to open a v4l2 device for writing with PyAV I always get the error

    container = av.open(outdev, mode="w", format="v4l2")
  File "av/container/core.pyx", line 363, in av.container.core.open
  File "av/container/core.pyx", line 145, in av.container.core.Container.__cinit__
ValueError: Could not determine output format