MNT: Remove internal usage of the 'backend' key in rcParams

Author: timhoffm

lib/matplotlib/__init__.py

@@ -799,18 +799,21 @@ def __setitem__(self, key, val):
 
     def __getitem__(self, key):
         if key == "backend":
-            # In theory, this should only ever be used after the global rcParams
-            # has been set up, but better be safe e.g. in presence of breakpoints.
-            if (self is globals().get("rcParams")
-                    and RcParams._backend is rcsetup._auto_backend_sentinel):
-                from matplotlib import pyplot as plt
-                plt.switch_backend(rcsetup._auto_backend_sentinel)
-            return RcParams._backend
+            return self._get_backend_resolved()
         return self._get(key)
 
-    def _get_backend_or_none(self):
-        """Get the requested backend, if any, without triggering resolution."""
-        backend = RcParams._backend
+    @classmethod
+    def _get_backend_resolved(cls):
+        """Get the backend, resolving the auto-sentinel if needed."""
+        if cls._backend is rcsetup._auto_backend_sentinel:
+            from matplotlib import pyplot as plt
+            plt.switch_backend(rcsetup._auto_backend_sentinel)
+        return cls._backend
+
+    @classmethod
+    def _get_backend_or_none(cls):
+        """Get the current backend or None."""
+        backend = cls._backend
         return None if backend is rcsetup._auto_backend_sentinel else backend
 
     def __repr__(self):
@@ -1292,14 +1295,14 @@ def use(backend, *, force=True):
         # value which will be respected when the user finally imports
         # pyplot
         else:
-            rcParams['backend'] = backend
+            rcParams._backend = backend
     # if the user has asked for a given backend, do not helpfully
     # fallback
     rcParams['backend_fallback'] = False
 
 
 if os.environ.get('MPLBACKEND'):
-    rcParams['backend'] = os.environ.get('MPLBACKEND')
+    rcParams._backend = os.environ.get('MPLBACKEND')
 
 
 def get_backend(*, auto_select=True):
@@ -1326,13 +1329,9 @@ def get_backend(*, auto_select=True):
     matplotlib.use
     """
     if auto_select:
-        return rcParams['backend']
+        return rcParams._get_backend_resolved()
     else:
-        backend = rcParams._get('backend')
-        if backend is rcsetup._auto_backend_sentinel:
-            return None
-        else:
-            return backend
+        return rcParams._get_backend_or_none()
 
 
 def interactive(b):

lib/matplotlib/pyplot.py

@@ -73,7 +73,7 @@
     FigureCanvasBase, FigureManagerBase, MouseButton)
 from matplotlib.figure import Figure, FigureBase, figaspect
 from matplotlib.gridspec import GridSpec, SubplotSpec
-from matplotlib import rcsetup, rcParamsDefault, rcParamsOrig
+from matplotlib import rcsetup
 from matplotlib.artist import Artist
 from matplotlib.axes import Axes
 from matplotlib.axes import Subplot  # noqa: F401
@@ -385,10 +385,10 @@ def _get_backend_mod() -> type[matplotlib.backend_bases._Backend]:
     This is currently private, but may be made public in the future.
     """
     if _backend_mod is None:
-        # Use rcParams._get("backend") to avoid going through the fallback
+        # Use rcParams._backend to avoid going through the fallback
         # logic (which will (re)import pyplot and then call switch_backend if
         # we need to resolve the auto sentinel)
-        switch_backend(rcParams._get("backend"))
+        switch_backend(rcParams._backend)
     return cast(type[matplotlib.backend_bases._Backend], _backend_mod)
 
 
@@ -432,16 +432,13 @@ def switch_backend(newbackend: str) -> None:
                 _log.debug("Skipping backend candidate %r as loading failed.",
                            candidate, exc_info=True)
                 continue
-            else:
-                rcParamsOrig['backend'] = candidate
-                return
+            return
         else:
             # Switching to Agg should always succeed; if it doesn't, let the
             # exception propagate out.
             switch_backend("agg")
-            rcParamsOrig["backend"] = "agg"
             return
-    old_backend = rcParams._get('backend')  # get without triggering backend resolution
+    old_backend = rcParams._backend
 
     module = backend_registry.load_backend_module(newbackend)
     canvas_class = module.FigureCanvas
@@ -530,7 +527,7 @@ def draw_if_interactive() -> None:
         except im.PackageNotFoundError:
             pass
 
-    rcParams['backend'] = rcParamsDefault['backend'] = newbackend
+    rcParams._backend = newbackend
     _backend_mod = backend_mod
     for func_name in ["new_figure_manager", "draw_if_interactive", "show"]:
         globals()[func_name].__signature__ = inspect.signature(

lib/matplotlib/tests/test_backends_interactive.py

@@ -168,7 +168,7 @@ def _test_interactive_impl():
     })
 
     mpl.rcParams.update(json.loads(sys.argv[1]))
-    backend = plt.rcParams["backend"].lower()
+    backend = plt.rcParams.get_backend_resolved().lower()
 
     if backend.endswith("agg") and not backend.startswith(("gtk", "web")):
         # Force interactive framework setup.
@@ -296,7 +296,7 @@ def _test_thread_impl():
     # the figure
     canvas = fig.canvas
     plt.close()  # backend is responsible for flushing any events here
-    if plt.rcParams["backend"].lower().startswith("wx"):
+    if plt.rcParams._get_backend_resolved().lower().startswith("wx"):
         # TODO: debug why WX needs this only on py >= 3.8
         canvas.flush_events()