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Eventually #5414 will fix this altogether, but this is fine for now.

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I would change the text to something like

"Building Matplotlib font cache by running fc-list. This could take a few minutes."

Maybe "Matplotlib is building {cachename} using fc-list. This may take a moment".

I prefer "moment" over "minutes" because it is ambiguous and thus a bit future-proof. Each person's perception of a moment is shaped by the computer they are using, so it is a bit more flexible, too.

backported to 2.x as c4a94e5 and v1.5.x as 0823022 (because I did not read the milestone carefully enough).

One thing I don't understand: why is it giving this warning every time I load matplotlib? Does it build the font cache every time?

It should only be rebuilding each time if it doesn't have write permissions to ~/.matplotlib (or ~/.cache/matplotlib on Linux).

@mdboom, thanks for your quick reply. I am using Ubuntu, my user owns ~/.cache and ~/.cache/matplotlib, and there's indeed fontList.cache file there, but it dates to April last year. Importing matplotlib.pyplot does not seem to touch that file.

Do you own that file too and have user write access to it?

Sure:

peleg@lati::~/.cache/matplotlib$ ls -la
total 308
drwxrwxr-x  3 peleg peleg   4096 Jan 11 18:43 .
drwx------ 50 peleg peleg   4096 Jan 11 11:08 ..
-rw-rw-r--  1 peleg peleg 161444 Apr  6  2015 fontList.cache
-rw-rw-r--  1 peleg peleg 138010 Apr 24  2015 fontList.py3k.cache
drwxrwxr-x  2 peleg peleg   4096 Apr  6  2015 tex.cache

(owner write access...)

Which version are you using 1.5.x and 2.x uses a binary font cache but
on master it is changed to using a json fine see #5021

I would suggest moving those files out of the way and try again if you are using 1.5.1 or on the 2.x branch.

I am using matplotlib 1.5.1, just downloaded via pip.

I have removed the files as you suggested, and after they were created again (during the first time I've imported matplotlib.pyplot), the warning does not appear any more. Thanks for your help!

I run into similar behavior when opening both my system matplotlib of 1.3.1 and my installed version of 1.5.1. Apparently the older file format with matplotlib 1.3.1 always causes matplotlib 1.5.1 to rescan, but not update the file.

I'm glad this warning is here as it means I know to wait for it to run. @pelegm I found that if I deleted everything in ~/.matplotlib (I'm on OSX) it generated once and I've not had the message since.