[MNT]: pyplot.xkcd() leads to findfont errors under Windows

[bkasel]

Summary

any plot with the xkcd style pyplot.xkcd() generates a bunch of errors from the findfont function under Windows even when fonts like "Comic Neue" are installed. I also regenerated the font cache and copied the font files into the matplotlib site-packages font folder, but this did not help. (I am using mpl 3.8.2 on python 3.11)

I am unsure if this is an issue under other OSs but I have the same issue under Win10 and Win11

Proposed fix

In the end I modified the pyplot.py file in the xkcd function, to get rid of the errors

    rcParams.update({
        'font.font-family': ['xkcd', 'xkcd Script', 'Comic Neue', 'Comic Sans MS'],
...

to

   rcParams.update({
       'font.sans-serif': ['xkcd', 'xkcd Script', 'Comic Neue', 'Comic Sans MS'],
...

[story645]

I'd assume this would be due to #27339 but the only change there was removing humor sans.

[QuLogic]

I think this is older than that; the caching on the warning seems either very specific, or non-existent, as it seems to trigger at all draws (e.g., pan or resize, and a bunch of warnings are printed.)

[story645]

or non-existent, as it seems to trigger at all draws

That's what I thought was going on w/ the findfont warning - is why I started #26602

Also I don't know if it's a windows specific thing, but I think I have to delete my fontcache to force a rebuild.

[StefRe]

These warnings are caused by _find_fonts_by_props introduced in #20740 with matplotlib 3.6.0.

This function searches the complete list of font.family:

matplotlib/lib/matplotlib/font_manager.py

Line 1351 in 0a4efbe

for family in prop.get_family():

implicitly assuming that all entries are family names. If this list also contains concrete font names, a warning will be issued if a font name is not found. Prior to v3.6.0, only the existing font files from the font cache were reviewed in _findfont_cached.

See the following example:

import matplotlib as mpl
import matplotlib.pyplot as plt

with mpl.rc_context({'font.family': ["monospace", "DejaVu Sans Mono", "Bitstream Vera Sans Mono"]}):
    fig = plt.figure()
    fig.text(0.4, 0.5, "mimi\nimim", size=40)

Starting with v3.6.0 it returns the warnings WARNING:matplotlib.font_manager:findfont: Font family 'Bitstream Vera Sans Mono' not found.. No warnings occurred up to v3.5.3.

So to fix this concrete issue it would indeed be sufficient to set font.family to 'sans-serif' (just in case it has been changed from the default before) and then set the list of concrete fonts to 'font.sans-serif' as @bkasel already found out. This doesn't address, however, the underlying root cause.

[story645]

If this list also contains concrete font names, a warning will be issued if a font name is not found.

My preference would be to allow font-names as part of finding family names (or control w/ a flag that defaults to permissive), what am I missing for why that's a bad idea?

[StefRe]

My preference would be to allow font-names as part of finding family names (...), what am I missing for why that's a bad idea?

I didn't say that font.family must not contain concrete font names (sorry if I phrased that in a misleading way). On the contrary, I think it might be sufficient to just emit the warning only for font families, not for concrete fonts (but I didn't check yet if this completely restores the old behavior):

@@ -1367,7 +1367,7 @@ class FontManager:
                         "none of the following families were found: %s",
                         family, ", ".join(self._expand_aliases(family))
                     )
-                else:
+                elif family in ["serif", "sans-serif", "cursive", "fantasy", "monospace"]:
                     _log.warning("findfont: Font family %r not found.", family)

[story645]

didn't say that font.family must not contain concrete font names

I think I'm the one who phrased things weird, I was just asking if we could expand find font family to not raise a warning if the font exists

[StefRe]

... to not raise a warning if the font exists

I guess it should read "if the font doesn't exist". If the font exists then findfont will not raise and no warning will be issued. But I think that we're both talking about the same thing.

[story645]

But I think that we're both talking about the same thing.

Probably - what I'm getting at is basically that when it comes to not found error messages, we should be assuming something like this relation for the rcparams: $font.[serif, fantasy, etc] \subset font.family$

so that if updating $font.sans-serif$ doesn't raise a warning, than updating $font.family$ also shouldn't. I know technically $family \neq font$ but there are lots of places in our examples were we pass in a font to a family parameter so we're kinda using them interchangeably anyway.