Private helper to get requested backend without triggering resolution.

anntzer · anntzer · commit 2d8505b618b5 · 2022-06-13T22:33:17.000+02:00
... avoiding having to spell out the `dict.__getitem__` dance, which is
really an implementation detail of rcParams.
diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
@@ -678,6 +678,11 @@ def __getitem__(self, key):
 
         return dict.__getitem__(self, key)
 
+    def _get_backend_or_none(self):
+        """Get the requested backend, if any, without triggering resolution."""
+        backend = dict.__getitem__(self, "backend")
+        return None if backend is rcsetup._auto_backend_sentinel else backend
+
     def __repr__(self):
         class_name = self.__class__.__name__
         indent = len(class_name) + 1
@@ -1129,9 +1134,8 @@ def use(backend, *, force=True):
     matplotlib.get_backend
     """
     name = validate_backend(backend)
-    # we need to use the base-class method here to avoid (prematurely)
-    # resolving the "auto" backend setting
-    if dict.__getitem__(rcParams, 'backend') == name:
+    # don't (prematurely) resolve the "auto" backend setting
+    if rcParams._get_backend_or_none() == name:
         # Nothing to do if the requested backend is already set
         pass
     else:
diff --git a/lib/matplotlib/backends/qt_compat.py b/lib/matplotlib/backends/qt_compat.py
@@ -55,15 +55,10 @@
     QT_API = QT_API_PYSIDE2
 # Otherwise, check the QT_API environment variable (from Enthought).  This can
 # only override the binding, not the backend (in other words, we check that the
-# requested backend actually matches).  Use dict.__getitem__ to avoid
+# requested backend actually matches).  Use _get_backend_or_none to avoid
 # triggering backend resolution (which can result in a partially but
 # incompletely imported backend_qt5).
-elif (
-        isinstance(dict.__getitem__(mpl.rcParams, "backend"), str) and
-        dict.__getitem__(mpl.rcParams, "backend").lower() in [
-            "qt5agg", "qt5cairo"
-        ]
-):
+elif (mpl.rcParams._get_backend_or_none() or "").lower().startswith("qt5"):
     if QT_API_ENV in ["pyqt5", "pyside2"]:
         QT_API = _ETS[QT_API_ENV]
     else:
diff --git a/lib/matplotlib/pyplot.py b/lib/matplotlib/pyplot.py
@@ -2195,7 +2195,7 @@ def polar(*args, **kwargs):
 # requested, ignore rcParams['backend'] and force selection of a backend that
 # is compatible with the current running interactive framework.
 if (rcParams["backend_fallback"]
-        and dict.__getitem__(rcParams, "backend") in (
+        and rcParams._get_backend_or_none() in (
             set(_interactive_bk) - {'WebAgg', 'nbAgg'})
         and cbook._get_running_interactive_framework()):
     dict.__setitem__(rcParams, "backend", rcsetup._auto_backend_sentinel)
