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

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.

Author: timhoffm

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):

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]: ...

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({