Lazy load matplotlib to avoid import locks about python-igraph

[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.