MNT: Don't rely on RcParams being a dict subclass in internal code

timhoffm · timhoffm · commit 01ceeef0b28e · 2024-08-15T17:47:11.000+02:00
Eventually, we want to be able to remove the dict subclassing from RcParams, which will allow better initialization and handling. We've publically announced that people should not rely on dict in https://matplotlib.org/stable/api/prev_api_changes/api_changes_3.7.0.html#rcparams-type This is an internal cleanup step to remove the dict-assumption and a preparation for further refactoring.
diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
@@ -712,6 +712,35 @@ def _get(self, key):
         """
         return dict.__getitem__(self, key)
 
+    def _update_raw(self, other_params):
+        """
+        Directly update the data from *other_params*, bypassing deprecation,
+        backend and validation logic on both sides.
+
+        This ``rcParams._update_raw(params)`` replaces the previous pattern
+        ``dict.update(rcParams, params)``.
+
+        Parameters
+        ----------
+        other_params : dict or `.RcParams`
+            The input mapping from which to update.
+        """
+        if isinstance(other_params, RcParams):
+            other_params = dict.items(other_params)
+        dict.update(self, other_params)
+
+    def _ensure_has_backend(self):
+        """
+        Ensure that a "backend" entry exists.
+
+        Normally, the default matplotlibrc file contains *no* entry for "backend" (the
+        corresponding line starts with ##, not #; we fill in _auto_backend_sentinel
+        in that case.  However, packagers can set a different default backend
+        (resulting in a normal `#backend: foo` line) in which case we should *not*
+        fill in _auto_backend_sentinel.
+        """
+        dict.setdefault(self, "backend", rcsetup._auto_backend_sentinel)
+
     def __setitem__(self, key, val):
         try:
             if key in _deprecated_map:
@@ -961,24 +990,17 @@ def rc_params_from_file(fname, fail_on_error=False, use_default_template=True):
     return config
 
 
-# When constructing the global instances, we need to perform certain updates
-# by explicitly calling the superclass (dict.update, dict.items) to avoid
-# triggering resolution of _auto_backend_sentinel.
 rcParamsDefault = _rc_params_in_file(
     cbook._get_data_path("matplotlibrc"),
     # Strip leading comment.
     transform=lambda line: line[1:] if line.startswith("#") else line,
     fail_on_error=True)
-dict.update(rcParamsDefault, rcsetup._hardcoded_defaults)
-# Normally, the default matplotlibrc file contains *no* entry for backend (the
-# corresponding line starts with ##, not #; we fill on _auto_backend_sentinel
-# in that case.  However, packagers can set a different default backend
-# (resulting in a normal `#backend: foo` line) in which case we should *not*
-# fill in _auto_backend_sentinel.
-dict.setdefault(rcParamsDefault, "backend", rcsetup._auto_backend_sentinel)
+rcParamsDefault._update_raw(rcsetup._hardcoded_defaults)
+rcParamsDefault._ensure_has_backend()
+
 rcParams = RcParams()  # The global instance.
-dict.update(rcParams, dict.items(rcParamsDefault))
-dict.update(rcParams, _rc_params_in_file(matplotlib_fname()))
+rcParams._update_raw(rcParamsDefault)
+rcParams._update_raw(_rc_params_in_file(matplotlib_fname()))
 rcParamsOrig = rcParams.copy()
 with _api.suppress_matplotlib_deprecation_warning():
     # This also checks that all rcParams are indeed listed in the template.
@@ -1190,7 +1212,7 @@ def rc_context(rc=None, fname=None):
             rcParams.update(rc)
         yield
     finally:
-        dict.update(rcParams, orig)  # Revert to the original rcs.
+        rcParams._update_raw(orig)  # Revert to the original rcs.
 
 
 def use(backend, *, force=True):
diff --git a/lib/matplotlib/__init__.pyi b/lib/matplotlib/__init__.pyi
@@ -70,6 +70,10 @@ class RcParams(dict[str, Any]):
     def __init__(self, *args, **kwargs) -> None: ...
     def _set(self, key: str, val: Any) -> None: ...
     def _get(self, key: str) -> Any: ...
+
+    def _update_raw(self, other_params: dict | RcParams) -> None: ...
+
+    def _ensure_has_backend(self) -> None: ...
     def __setitem__(self, key: str, val: Any) -> None: ...
     def __getitem__(self, key: str) -> Any: ...
     def __iter__(self) -> Generator[str, None, None]: ...
diff --git a/lib/matplotlib/pyplot.py b/lib/matplotlib/pyplot.py
@@ -410,8 +410,7 @@ def switch_backend(newbackend: str) -> None:
             switch_backend("agg")
             rcParamsOrig["backend"] = "agg"
             return
-    # have to escape the switch on access logic
-    old_backend = dict.__getitem__(rcParams, 'backend')
+    old_backend = rcParams._get('backend')  # get without triggering backend resolution
 
     module = backend_registry.load_backend_module(newbackend)
     canvas_class = module.FigureCanvas
@@ -841,7 +840,7 @@ def xkcd(
             "xkcd mode is not compatible with text.usetex = True")
 
     stack = ExitStack()
-    stack.callback(dict.update, rcParams, rcParams.copy())  # type: ignore[arg-type]
+    stack.callback(rcParams._update_raw, rcParams.copy())  # type: ignore[arg-type]
 
     from matplotlib import patheffects
     rcParams.update({
