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add documentation for reloading font cache #26453

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27 changes: 27 additions & 0 deletions galleries/users_explain/text/fonts.py
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Expand Up @@ -196,6 +196,33 @@
Each glyph of the string is rendered using the first font in the list that
contains that glyph.

Loading new fonts
-----------------

All fonts that are installed in your system will become available to the
`.FontManager`, as well as the default fonts that ship with Matplotlib. The latter
fonts are located in the directory ``'matplotlib\mpl-data\fonts\ttf'``.
It is also possible to add fonts to this directory, or to `allocate a custom
directory
<https://matplotlib.org/stable/api/font_manager_api.html#matplotlib.font_manager.findSystemFonts>`_
that contains any fonts you wish to add to the `.FontManager`.

In either case, the `.FontManager` will only find the new fonts after the font cache
has been rebuilt following their addition . You may need to rebuild the cache
yourself if your new fonts cannot be found by Matplotlib.
The cache can be rebuilt by using :func:`matplotlib.font_manager.load_fontmanager`::
import matplotlib as mpl
mpl.font_manager.load_fontmanager(try_read_cache=False)
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I don't think that this will actually work?

This will make a new font manager and return it to you, but it will not change the singleton that is used by the rest of the library or update the findfont and get_font_names held at the module level.

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My testing procedure for this was:

  • add a new font to mpl-data\fonts\ttf
  • run a short plot with a text instance with family=new font
  • see that the text doesn't update to the new font after multiple runs
  • run the font_manager.load_fontmanager code
  • run the plot again

Now the text is now set to the new font and a new entry in the font cache is present. The findfont and get_font_names methods both return the new font.
I did notice however, that the before and after fontmanager instances were the same object with the same ID.

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... ok, I do not understand why that works 🤯

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ok, question: are you running this all in the same process or each in its own Python process?

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Each in their own separate process.


Alternatively, you can expose the location of your font cache using
:func:`matplotlib.get_cachedir`::
import matplotlib as mpl
mpl.get_cachedir()
'/home/darren/.cache/matplotlib'

You can then manually delete the cache file ``'fontlist<version>.json'``. The new fonts
will be available the next time the script is run.

A majority of this work was done by Aitik Gupta supported by Google Summer of
Code 2021.
"""
11 changes: 7 additions & 4 deletions lib/matplotlib/font_manager.py
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Expand Up @@ -1456,10 +1456,10 @@ def _findfont_cached(self, prop, fontext, directory, fallback_to_default,
if rebuild_if_missing:
_log.info(
'findfont: Found a missing font file. Rebuilding cache.')
new_fm = _load_fontmanager(try_read_cache=False)
new_fm = load_fontmanager(try_read_cache=False)
# Replace self by the new fontmanager, because users may have
# a reference to this specific instance.
# TODO: _load_fontmanager should really be (used by) a method
# TODO: load_fontmanager should really be (used by) a method
# modifying the instance in place.
vars(self).update(vars(new_fm))
return self.findfont(
Expand Down Expand Up @@ -1558,7 +1558,10 @@ def get_font(font_filepaths, hinting_factor=None):
)


def _load_fontmanager(*, try_read_cache=True):
def load_fontmanager(*, try_read_cache=True):
"""
Re-loads the font cache by generating a new `.FontManager` instance.
"""
fm_path = Path(
mpl.get_cachedir(), f"fontlist-v{FontManager.__version__}.json")
if try_read_cache:
Expand All @@ -1576,6 +1579,6 @@ def _load_fontmanager(*, try_read_cache=True):
return fm


fontManager = _load_fontmanager()
fontManager = load_fontmanager()
findfont = fontManager.findfont
get_font_names = fontManager.get_font_names
1 change: 1 addition & 0 deletions lib/matplotlib/font_manager.pyi
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Expand Up @@ -129,3 +129,4 @@ def findfont(
rebuild_if_missing: bool = ...,
) -> str: ...
def get_font_names() -> list[str]: ...
def load_fontmanager(try_read_cache: bool = ...) -> FontManager: ...