v4l2 throws a TypeError when imported in Python 3.5

[ronbarak]

When importing v4l2, I get:

openstack@prclnx04:~/python/v4l2$ python3 -c "import v4l2"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/local/lib/python3.5/dist-packages/v4l2.py", line 197, in <module>
    ) = range(1, 9) + [0x80]
TypeError: unsupported operand type(s) for +: 'range' and 'list'

And the relevant lines in /usr/local/lib/python3.5/dist-packages/v4l2.py are:

openstack@prclnx04:~/python/v4l2$ cat /usr/local/lib/python3.5/dist-packages/v4l2.py | head -197 | tail -12
v4l2_buf_type = enum
(
    V4L2_BUF_TYPE_VIDEO_CAPTURE,
    V4L2_BUF_TYPE_VIDEO_OUTPUT,
    V4L2_BUF_TYPE_VIDEO_OVERLAY,
    V4L2_BUF_TYPE_VBI_CAPTURE,
    V4L2_BUF_TYPE_VBI_OUTPUT,
    V4L2_BUF_TYPE_SLICED_VBI_CAPTURE,
    V4L2_BUF_TYPE_SLICED_VBI_OUTPUT,
    V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY,
    V4L2_BUF_TYPE_PRIVATE,
) = range(1, 9) + [0x80]

---------------------------------
Notes:
1. vl42 was installed using:

openstack@prclnx04:~/python/v4l2$ sudo -H python3 -m pip install v4l2 --upgrade
Requirement already up-to-date: v4l2 in /usr/local/lib/python3.5/dist-packages

2. I use Python3.5 on Ubuntu 16.04

[ronbarak (p-launchpad-comverse)]

If I change the following two lines:

openstack@prclnx04:~/python/v4l2$ grep -n "i in range" "/usr/local/lib/python3.5/dist-packages/v4l2.py" | grep range
197:) = [i+0x80 for i in range(1, 10)]
249:) = [i+2 for i in range(0, 5)]

The import is successful.

[ash (sersorrel)]

That's probably not the correct fix: `range(...)` in Python 2 is equivalent to `list(range(...))` in Python 3, so those lines should probably be

    ) = list(range(1, 10)) + [0x80]

and

    ) = list(range(0, 5)) + [2]

instead.

[Justin Gottula (jgottula)]

Both of the fixes proposed in the comments above are not *quite* right.

------------------------------------------------------------

ronbarak's proposal in #1 results in the wrong values for the enumerations, because the `+ [0x80]` and `+ [2]` are not modifiers that affect all of the enum values; but rather, a way of separately setting the last member of the enum to a different (non-sequential) value.

- For `v4l2_buf_type`, the enum values go sequentially from 1 to 8, and then the last enum member, `V4L2_BUF_TYPE_PRIVATE`, is separately set to 0x80.

- For `v4l2_priority`, the enum values go sequentially from 0 to 3, and then the last enum member, `V4L2_PRIORITY_DEFAULT`, is separately set to 2 (the same value as `V4L2_PRIORITY_INTERACTIVE`).

------------------------------------------------------------

The following links to the V4L2 C headers should help clarify the situation.

- `v4l2_buf_type`: https://git.linuxtv.org/v4l-utils.git/tree/include/linux/videodev2.h?id=v4l-utils-1.14.2#n132

- `v4l2_priority`: https://git.linuxtv.org/v4l-utils.git/tree/include/linux/videodev2.h?id=v4l-utils-1.14.2#n381

------------------------------------------------------------

Josh Holland's comment (#2) is on the mark WRT the correct way to fix the core Python 2+3 compatibility issue; but the code lines provided appear to have re-used the wrong `start` and `stop` values for the `range()` statements (taken from comment #1).

So rather than

  ) = list(range(1, 10)) + [0x80]

the `v4l2_buf_type` line ought to be

  ) = list(range(1, 9)) + [0x80]

instead; and rather than

  ) = list(range(0, 5)) + [2]

the `v4l2_priority` line ought to be

  ) = list(range(0, 4)) + [2]

instead.

------------------------------------------------------------

I've attached a patch with a proper working fix. I've tested it and ensured that it works without error on both Python 2 and Python 3, and I've verified that the enumeration values are correct.

Here's a branch that I created with the fix integrated into it, over a month ago:
https://code.launchpad.net/~jgottula/python-v4l2/fix-for-bug-1664158

I've submitted requests for that branch to be reviewed and merged by the python-v4l2 developers *twice* in the past couple of months; there has been no response or action whatsoever. This, combined with the fact that this trivial bug report has been open (in "new" status, with tumbleweeds blowing by) for over a year now, and the fact that this repository has apparently had no commits in the past ~8 years (the last was on 2010-07-21), leads me to wonder whether the developers in charge of this project/repository are deceased or otherwise incapacitated in some manner...

[Maxime Kawawa-Beaudan (maximejkb)]

Hi Justin,

Thank you for that patch. I'm using it for a project with an Omega Onion 2+, a single-board computer similar to a Raspberry Pi, which has an attached USB camera. After downloading your patch, I'm able to build the module and import v4l2 at the top of my Python 3.6.0 file. Now I have another problem, and I'm not sure it's related to this patch, but this is the first and only version of the Python V4L2 API I've used, so I figured I'd ask here. I'm trying to run the basic sample code from the V4L2 project description:

>>> import v4l2
>>> import fcntl
>>> vd = open('/dev/video0', 'rw')
>>> cp = v4l2.v4l2_capability()
>>> fcntl.ioctl(vd, v4l2.VIDIOC_QUERYCAP, cp)

I've tried running this both from the REPL and as a file, and whatever I do, I get the error:

Traceback (most recent call last):
  File "Camera.py", line 7, in <module>
    fcntl.ioctl(vd, v4l2.VIDIOC_QUERYCAP, cp)
OSError: [Errno 25] Not a tty

I'm able to open /dev/video0, and I'm able to query the camera for an image with fswebcam, for example. I've also been using V4L2 up until now, just not in Python -- I was using V4L2 to change the camera's exposure and other settings, and using mjpg-streamer to capture still images. I want to be able to capture still images without the latency from mjpg-streamer and more reliably. The camera responds to V4L2, as v4l2-ctl --list-devices yields:

HD USB Camera (usb-101c0000.ehci-1):
 /dev/video0

Any help would be much appreciated.

[John McMaster (johndmcmaster)]

It sounds like there were a few proposals but they were not merged in? IMHO it would be better to have one of these sub-optimal patches rather than no action. Please let me know if I can do something to get this fixed. Thanks!