The problem#

The monitor-lock.py script in my previous blog post uses python-xlib, which currently mainly relies on manually porting Xlib functions to Python. This is why it is missing the barrier-related functions I needed in that post. There is work on automating this process, but it appears to be abandoned. I started trying to pick up where they had left off before finding the python-xcffib project which provides auto-generated bindings for libxcb and therefore gives full support for interacting with X at a low level from Python.

python-xcffib (named after the cffi library it uses for binding to the C XCB library) gives a slightly lower-level API than python-xlib, but they are both fairly thin wrappers over the X protocol, so the differences are minor. It was fairly straightforward to port my script from the previous post to use python-xcffib, available as monitor-lock-xcb.py.

Unfortunately, I ran into a bug in python-xcffib:

Traceback (most recent call last):
...
  File "./monitor-lock-xcb.py", line 38, in main
    devices = conn.xinput.XIQueryDevice(xcffib.xinput.Device.AllMaster).reply().infos
...
  File "/usr/lib/python3/dist-packages/xcffib/__init__.py", line 139, in _resize
    assert self.size + increment <= self.known_max
AssertionError

The solution#

I've submitted the fix upstream, so most likely you will not encounter this error. Updating to the latest version (after v0.8.1) should be sufficient to fix the problem.

The fix I applied was to modify the module's __init__.py (the location, which may be different on your machine, is in the stack trace). Specifically, on line 108 in the function Unpacker.unpack(), in the call to struct.calcsize(), change fmt to "=" + fmt.

The details#