Matplotlib intermittently prints during some operations

[m-roberts]

This seems to be most evident when using the CLI:

pi@pi-top:~ $ pi-top devices hub
Matplotlib is building the font cache; this may take a moment.

Interestingly, this does not appear to be related to our explicit use of matplotlib.

from matplotlib.font_manager import FontManager
f = FontManager()

The creation of this object is what triggers this message - however, it is on a 5s timer.

pi@pi-top:~ $ time !!
time python3 -c "from matplotlib.font_manager import FontManager; f = FontManager()"

real	0m4.416s
user	0m4.273s
sys	0m0.827s

Given that the time that this takes is ~4.5s on a Pi 4 anyway, it seems that this is something that will continue to occur. There is no API for controlling the behaviour of this class.

Source:

# Delay the warning by 5s.
timer = Timer(5, lambda: _log.warning(
    'Matplotlib is building the font cache; this may take a moment.'))
timer.start()
try:
    for fontext in ["afm", "ttf"]:
        for path in [*findSystemFonts(paths, fontext=fontext),
                     *findSystemFonts(fontext=fontext)]:
            try:
                self.addfont(path)
            except OSError as exc:
                _log.info("Failed to open font file %s: %s", path, exc)
            except Exception as exc:
                _log.info("Failed to extract font properties from %s: "
                          "%s", path, exc)
finally:
    timer.cancel()

[m-roberts]

One solution for this would be to package up a modified version of this package which has different behaviour (perhaps a longer timer?).