Lazy load matplotlib to avoid import locks

[PhilipVinc]

On a SLURM cluster I've seen some hangs due to igraph loading matplotlib

TimeoutError: Lock error: Matplotlib failed to acquire the following lock file:
    /dev/shm/.cache-sdash/matplotlib/fontlist-v330.json.matplotlib-lock
This maybe due to another process holding this lock file.  If you are sure no
other Matplotlib process is running, remove this file and try again.
    with cbook._lock_path(filename), open(filename, 'w') as fh:
  File "/mnt/sw/nix/store/gpkc8q6zjnp3n3h3w9hbmbj6gjbxs85w-python-3.10.10-view/lib/python3.10/contextlib.py", line 135, in __enter__
    raise TimeoutError("""\

Unfortunately, it is not under my power to uninstall matplotlib because it is installed in some shared space. Ideally, igraph would load it lazily only if needed.

[iosonofabio]

Thanks for reporting. This is a site installation issue so there is a fundamental problem out of our control. Nonetheless, you could:

• use a per-user Python interpreter + packages
• use PYTHONPATH just for matplotlib
• mock it

We could also lazy load like you suggested. I'm really busy these days, do you think you could open a PR? I'd be happy to assist

[ntamas]

The error message that you see happens when multiple processes are using Matplotlib and they all want to write to ~/.matplotlib/fontlist-v330.json at the same time. This file is built the first time Matplotlib is invoked on the system, so you can work around the problem by pre-populating the Matplotlib font cache. IMHO lazy-loading Matplotlib wouldn't solve the problem as it could still happen that two processes would want to lazy-load Matplotlib at the same time.

[ntamas]

Another thing that you could try is to add a random delay to the startup of your process to decrease the chance of two copies of your project trying to import matplotlib at the same time.

[szhorvat]

I had a similar issue with a different software that also created certain files on first start. I worked around it by logging in to the nodes of the HPC cluster where my code would run, and starting up a single instance of this software manually, so that the file could be created correctly.

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