Fix rc init when matplotlib is imported first

ultraplot/internals/rcsetup.py

@@ -734,6 +734,14 @@ def copy(self):
 if not hasattr(RcParams, "validate"):  # not mission critical so skip
     warnings._warn_ultraplot("Failed to update matplotlib rcParams validators.")
 else:
+
+    def _validator_accepts(validator, value):
+        try:
+            validator(value)
+            return True
+        except Exception:
+            return False
+
     _validate = RcParams.validate
     _validate["image.cmap"] = _validate_cmap("continuous")
     _validate["legend.loc"] = _validate_belongs(*LEGEND_LOCS)
@@ -752,6 +760,20 @@ def copy(self):
             _validate[_key] = functools.partial(_validate_color, alternative="auto")
         if _validator is getattr(msetup, "validate_color_or_inherit", None):
             _validate[_key] = functools.partial(_validate_color, alternative="inherit")
+        # Matplotlib may wrap fontsize validators in callable objects instead of
+        # exposing validate_fontsize directly. Detect these by behavior so custom
+        # shorthands like "med-large" remain valid regardless of import order.
+        if (
+            _key.endswith("size")
+            and _key not in FONT_KEYS
+            and _validator_accepts(_validator, "large")
+            and not _validator_accepts(_validator, "med-large")
+        ):
+            FONT_KEYS.add(_key)
+            if _validator_accepts(_validator, None):
+                _validate[_key] = _validate_or_none(_validate_fontsize)
+            else:
+                _validate[_key] = _validate_fontsize
     for _keys, _validator_replace in ((EM_KEYS, _validate_em), (PT_KEYS, _validate_pt)):
         for _key in _keys:
             _validator = _validate.get(_key, None)

ultraplot/tests/test_config.py

@@ -1,4 +1,8 @@
 import importlib
+import os
+import pathlib
+import subprocess
+import sys
 import threading
 from queue import Queue
 
@@ -212,3 +216,45 @@ def _reader():
     observed = [results.get() for _ in range(results.qsize())]
     assert observed, "No rcParams observations were recorded."
     assert all(value in allowed for value in observed)
+
+
+def _run_in_subprocess(code):
+    code = (
+        "import pathlib\n"
+        "import sys\n"
+        "sys.path.insert(0, str(pathlib.Path.cwd()))\n" + code
+    )
+    env = os.environ.copy()
+    env["MPLBACKEND"] = "Agg"
+    return subprocess.run(
+        [sys.executable, "-c", code],
+        capture_output=True,
+        text=True,
+        cwd=str(pathlib.Path(__file__).resolve().parents[2]),
+        env=env,
+    )
+
+
+def test_matplotlib_import_before_ultraplot_allows_rc_mutation():
+    """
+    Import order regression test for issue #568.
+    """
+    result = _run_in_subprocess(
+        "import matplotlib.pyplot as plt\n"
+        "import ultraplot as uplt\n"
+        "uplt.rc['figure.facecolor'] = 'white'\n"
+    )
+    assert result.returncode == 0, result.stderr
+
+
+def test_matplotlib_import_before_ultraplot_allows_custom_fontsize_tokens():
+    """
+    Ensure patched fontsize validators are active regardless of import order.
+    """
+    result = _run_in_subprocess(
+        "import matplotlib.pyplot as plt\n"
+        "import ultraplot as uplt\n"
+        "for key in ('axes.titlesize', 'figure.titlesize', 'legend.fontsize', 'xtick.labelsize'):\n"
+        "    uplt.rc[key] = 'med-large'\n"
+    )
+    assert result.returncode == 0, result.stderr