MNT: helper function for checking for presence of fonts in tests

[story645]

As mentioned in #27121, some of the tests could be cleaned up if there was a function to check for the presence of fonts.

I guess we really just need a helper in font_manager that does basically the same thing as findfont, but returns None (or raises) when the font of the requested family is not found, without ever trying to return a font from another family or performing fallback?

Originally posted by @anntzer in #27128 (comment)

[tacaswell]

I think we can close this with no action because findfont already has a keyword arguement that does this:

imPython 3.11.8 (main, Feb 12 2024, 14:50:05) [GCC 13.2.1 20230801]
Type 'copyright', 'credits' or 'license' for more information
IPython 8.22.2 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import matplotlib.font_manager as fm

In [2]: fm.findfont('noro')
findfont: Font family ['noro'] not found. Falling back to DejaVu Sans.
Out[2]: '/usr/lib/python3.11/site-packages/matplotlib/mpl-data/fonts/ttf/DejaVuSans.ttf'

In [3]: fm.findfont('noro', fallback_to_default=False)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[3], line 1
----> 1 fm.findfont('noro', fallback_to_default=False)

File /usr/lib/python3.11/site-packages/matplotlib/font_manager.py:1296, in FontManager.findfont(self, prop, fontext, directory, fallback_to_default, rebuild_if_missing)
   1292 ret = self._findfont_cached(
   1293     prop, fontext, directory, fallback_to_default, rebuild_if_missing,
   1294     rc_params)
   1295 if isinstance(ret, _ExceptionProxy):
-> 1296     raise ret.klass(ret.message)
   1297 return ret

ValueError: Failed to find font noro:style=normal:variant=normal:weight=normal:stretch=normal:size=10.0, and fallback to the default font was disabled

In [4]:

[story645]

ValueError is not the same as a true false b/c it requires wrapping the code in a try/catch. Which is why

but returns None

[story645]

I think @timhoffm's solution of testing helper works #28028 (comment)

[tacaswell]

I still think we should close this with no-action. If we need such a helper in the tests, then we should write it when we need it.

Having an unused helper around does not do any good and going through and adding code-churn to the tests just seems like unneeded work.

---

I do not think that returning a font or None is the best API here. It does let you do things like

if font := hypothetical_find_font(...):
    use(font)

so it makes sense for something like re.match where not finding something is "normal" as macth is "is there a match, and if so what is it?". On the other hand in our case if we ask for a font and can not find the font...we are out of luck and should raise as failing to find a font when you need one is truly exceptional!

If you want to run your own fallback you can write it like

for name in candidates:
    try:
        return fm.findfont(name, fallback_to_default=False)
    except ValueError:
        continue
else:
    raise ValueError(f"no fonts found of {candidates}")

Returning None also leaves open the risk of something like

font = hypothetical_find_font(...)
# for got to check!
...
# many LoC
...
function_that_uses(font) # got AttirubteError: None type has no attribute 
# ... in some other part of the code

so the source of the problem is well away from where the traceback will point you.